The conflict in Sudan's western Darfur region flared in 2003 when two rebel groups rose up against the government, accusing it of neglect.

Khartoum moved swiftly to crush the revolt by the Justice and Equality Movement (JEM) and the Sudan Liberation Army (SLA).

The government is widely accused of arming militias drawn from Arab tribes who have used scorched-earth tactics against the rebels' communities.

The militias, known as Janjaweed, are blamed for killings, widespread rape and abductions. Refugees describe them as ferocious gun-wielding men riding camels or horses who burn villages and steal whatever they can carry.

Khartoum has repeatedly denied any links to the Janjaweed, dismissing them as outlaws.

In March 2009 the International Criminal Court issued an arrest warrant for Sudan's President Omar Hassan al-Bashir for war crimes and crimes against humanity in Darfur. He has dismissed the charges.

Exact figures for the number of people killed in the conflict are hard to determine.

The United Nations says that as many as 300,000 may have died since 2003 - a figure disputed by Khartoum, which puts the figure closer to 10,000.

The violence has also driven up to 2.7 million people from their homes, according to the United Nations. Many are now living in squalid camps in Darfur and neighbouring Chad.

International Crisis Group says the removal of so many people from their homes appears to be part of a government policy of ethnic cleansing in a bid to cripple support for the rebel movements.

The displaced are mostly African farmers from the Fur, Zaghawa and Massaleit tribes. The Janjaweed come from Arabic-speaking pastoralist communities, which herd camels in northern Darfur and cattle in southern Darfur.

Both the farmers and the pastoralists are Muslim, dark-skinned and have intermarried for centuries.

The Zaghawa are also camel herders and have strong ties to Chad.

Darfur's population of 7 million is scattered over a harsh desert area the size of France.

The region, historically separate and long neglected by Khartoum, lacks basic infrastructure and social services. Experts say the motives for the rebellion were exclusion from political power, lack of roads, schools and water infrastructure.

The Darfur rebels' grievances are similar to those of some 30 armed groups in the south of the country and elsewhere.

Land used to belong to tribes - Darfur means 'the place of the Fur people'. There are at least 36 main tribes in the region.

Some of the Arab people felt left out of a system that gave more "dars" (districts) to non-Arab communities.

Traditionally, conflicts were settled with little or no violence by respected local councils. These were abolished by the Khartoum government after it came to power in a coup in 1989, leaving no mechanisms for resolving disputes peacefully.

The disbandment of the councils coincided with droughts and the encroachment of the expanding Sahara Desert, which has forced Arab herders from the north into competition over land with farmers based in villages.

To make matters worse, ethnic differences between the two groups - who used to co-exist by and large peacefully - have been exaggerated by local leaders in the battle over resources.

When Khartoum retaliated against Darfur's rebels it lit a powder keg.

The existence of oil in Darfur, revealed in 2005 when Sudan's ABCO corporation - which is partly owned by Swiss company Cliveden - began drilling, has led some analysts to wonder to what extent oil could be guiding Khartoum's actions in the region.

Some experts have suggested the oil reserves could act as an incentive to end the fighting. Drilling is risky during conflict, and if the oil can be extracted, there will be more wealth to go around.

In October 2007, the JEM rebel group said it had attacked Sudan's Defra oil field in the Kordofan region, neighbouring Darfur, killing 20 government soldiers and taking two foreign hostages - although Khartoum denied the raid. The rebels vowed to launch more assaults on oil installations across Sudan until Khartoum gave in to a string of demands.

Some more recent attacks on oil workers are believed to be the work of local tribesmen.

Many commentators say the conflict in Darfur has been exploited in the struggle for power within Sudan's Islamist movement. Hassan al-Turabi, an Islamist who was influential in the government but split in late 1999, has voiced his support for the rebels, increasing Khartoum's anger against him and the rebels.

The Sudanese government and rebels agreed to direct peace talks in Abuja, Nigeria, in November 2005. But progress was painfully slow.

Khartoum and SLA factional leader Minni Arcua Minnawi finally signed a deal in May 2006, but it was rejected by JEM and the leader of a rival SLA faction, Abdel Wahed Mohammed al-Nur.

Nur argued that the deal did not address some key demands of the Darfur people, including greater compensation for war victims, more political posts for the SLA, and greater SLA involvement in the protection of returning displaced people and disarmament of pro-government militias. Refugees rioted against the agreement in several camps and students protested in Khartoum.

Since the peace deal, the fighting has shifted from a more-or-less two-way conflict between central government and rebels to a more complex war also involving heavy fighting between various rebel factions.

Originally, the SLA united supporters from the Fur, Zaghawa and Massaleit tribes, but after the 2006 peace deal, it split increasingly along tribal lines. Minnawi is a Zaghawa, an ethnic group accounting for about 8 percent of Darfur's population. Nur is a Fur, which is the largest ethnic group in Darfur, comprising 30 percent of the population. JEM is mostly Zaghawa.

After the peace deal, the Sudanese military appeared to support Minnawi's side. His faction has been accused of using Janjaweed-like tactics, including raping and killing women from the Fur tribe. Nur's supporters have also been accused of gang-raping women for having Zaghawa husbands.

JEM has also splintered into many factions.

The United Nations and African Union said in May 2007 that their new focus was to unite the disparate rebel groups in readiness for fresh peace talks with Khartoum.

The rebel groups now number more than ten and, as new alliances form, it becomes ever harder for them to present a united front that can serve as a basis for negotiation.

Reports suggest Arab tribes in Darfur may also be losing patience with Khartoum. Some Arab communities are said to feel they have been used by the government to fight its battles, scapegoated for atrocities and then excluded from the political process.

One of the biggest Janjaweed groups defected from the government in October 2007, throwing its lot in with rebels it had previously been fighting. It later allied itself again with the government after securing concessions.

Peace negotiations between the government and Darfur rebels were left in ruins in May 2008 when JEM launched a shock attack on Khartoum.

Since then Qatar, representing the Arab League, has been working with the African Union and United Nations in mediating peace talks. The United Nations says this is the best hope yet for the region.

In October 2008, President Bashir launched a national initiative for peace with a forum to discuss the conflict, which was attended by AU representatives, as well as Egypt, Libya and Qatar, Sudanese political parties and civic groups. Rebel groups refused to attend.

Rebel leaders said they would not negotiate with Khartoum unless the government stopped its violence against civilians.

But in February 2009, JEM and Khartoum signed a goodwill agreement paving the way for possible peace talks. The agreement stopped short of a ceasefire, and hostilities between the two sides continue.

In early 2008, a hybrid United Nations-African Union force took over peacekeeping in Darfur from a purely AU force.

The 7,000-strong AU force had been massively overstretched and unable to quell the violence or protect civilians.

The U.N. Security Council authorised up to 26,000 troops and police for the new hybrid force, but as of March 2009, only 60 percent were on the ground.

Khartoum has been accused of impeding deployment and the international community has been blamed for not providing necessary equipment and funds.

A major challenge facing the new force is the lack of a peace agreement. U.N./AU-led efforts to revitalise the failed 2006 deal have foundered on the refusal of several major insurgent groups to take part in negotiations.

The deployment of UNAMID (United Nations-African Union Mission in Darfur) follows lengthy wrangling.

China took credit for eventually persuading Sudan to accept the additional peacekeepers. As a major buyer of Sudanese oil and supplier of arms, China is highly influential in Sudan and has come under international pressure to do more to stop the violence.

The Darfur conflict has also spilled over Sudan's western borders into Chad and Central African Republic, involving several armed groups that have forced people to flee their homes in all three countries.

Sudan and Chad accuse each other of helping rebel movements in one another's countries.

Chad also says Khartoum supports Janjaweed militias that attack African farming tribes inside its borders.

Chad's President Idriss Deby is from the Zaghawa tribe whose members live on both sides of the border and are among rebels fighting against Khartoum.

Around a quarter of a million Darfuris have fled to U.N.-run camps in Chad, while a smaller number of Chadian refugees have sought shelter in Darfur.

CAR's government also accuses Sudan of arming a coalition of rebels (the Union of Democratic Forces for Unity) who oppose President Francois Bozize and, in 2006, captured several towns in the northeast. Khartoum has denied any involvement.

Deby and Bashir signed a non-aggression pact in March 2008.

Until March 2009, some 16,370 aid workers in Darfur were providing relief to more than 4.7 million people, according to U.N. figures.

But Sudan expelled a number of large agencies after the International Criminal Court (ICC) issued an arrest warrant for President Bashir, who is charged with atrocities in Darfur.

The government accused them of passing information to the ICC - allegations they deny.

Some 13 international agencies and 3 local agencies were told to stop working in Darfur and other parts of northern and eastern Sudan.

Those who had their operating licences cancelled include Oxfam, Action Against Hunger, Save the Children, CARE, International Rescue Committee, Medecins Sans Frontieres Holland and the Norwegian Refugee Council.

U.N. officials warned the move would have a devastating impact on Darfuris.

For some time, aid workers in Darfur have faced increasing attacks and harassment by militia, rebels, bandits and even police, as well as bureaucratic obstacles.

Scores have been killed and many more injured. Aid agency compounds have been attacked, assets looted, staff threatened and clinics set on fire. Some groups have had to suspend operations in certain areas and hundreds of aid workers have left Darfur because of safety fears and restrictions on access.

U.N. agencies say rebel groups, militias and Chadian rebels have stolen humanitarian trucks for use in combat. Drivers have often been kidnapped. Hijackings in parts of western and northern Darfur have forced the United Nations to use helicopters.

The fragmentation of rebel groups has made it harder for aid groups to know who is in command, and to obtain guarantees of safe passage.

Relief workers also complain that Khartoum keeps them under surveillance and hampers their work with a host of administrative obstacles.

Aid workers have been arrested and senior U.N. officials prevented from visiting the region. Government obstruction and fighting have caused aid groups to withdraw staff and temporarily close their operations in Darfur on many occasions.

Conditions have also worsened for Darfur's internally displaced. Some have been forced to flee two or three times. More than 300,000 were uprooted in 2008 alone, according to the United Nations.

Increased fighting between rebel factions and government air attacks on rebels have exacerbated the risks to civilians. Villages continue to be burned, looted and bombed, and crops and livestock destroyed. Camps for the displaced have also been raided by armed groups.

Aid agencies warn that women face rising levels of sexual abuse - especially those who venture out of camps to collect firewood.

Malnutrition rates hover near emergency levels and hygiene in camps is poor. The United Nations has warned that insecurity is affecting healthcare as aid agencies are forced to scale back their work.

U.N. and non-governmental agencies say the humanitarian response risks breaking down entirely unless more is done to improve security for aid workers and civilians.

Human rights groups have accused Khartoum - under the aegis of Bashir - of torture and severe repression of political opposition and religious freedoms.

Washington has called the Darfur crisis "genocide", a term Khartoum rejects and European governments have been reluctant to use.

A U.N.-appointed commission concluded in 2005 that no genocide had taken place, but said there had been heinous war crimes no less serious than genocide. It also said individuals may have acted with genocidal intent.

In 2008, the ICC's chief prosecutor took the momentous step of charging Bashir with masterminding genocide in Darfur, killing 35,000 people and persecuting hundreds of thousands more. (Click here for a summary of the case.)

Prosecutor Luis Moreno-Ocampo also said 2.5 million people were subjected to a campaign of "rape, hunger and fear" in refugee camps.

The court has indicted Bashir for war crimes and crimes against humanity, but says it doesn't have sufficient grounds to include a count of genocide.

Bashir is the first head of state charged by an international court since Liberia's Charles Taylor and the former Yugoslavia's Slobodan Milosevic.

The U.N. Security Council has referred dozens of other names of Sudanese war crimes suspects to the ICC.

ICC prosecutors are keen to try a junior humanitarian affairs minister, Ahmed Haroun, and militia commander Ali Muhammed Ali Abd-al-Rahman, also known as Ali Kushayb.

Haroun was state minister of the interior at the height of the conflict. Moreno-Ocampo says evidence shows Haroun funded the Janjaweed from an unlimited budget and he was seen delivering arms to the militia.

Kushayb was allegedly seen giving orders to the Janjaweed, inspecting naked women before they were raped by men in military uniforms and participating in summary executions.

Frustrated at Khartoum's intransigence over Darfur, Washington has tightened sanctions against Sudan. The measures are mostly aimed at companies owned or controlled by the government, including firms in oil and petroleum export-related businesses.

The U.N. Security Council has imposed sanctions on four Sudanese accused of abuses in Darfur.