The Maoist crisis in the heartlands of India was described by Prime Minister Manmohan Singh in 2006 as India's biggest-ever internal security threat.

Maoist violence has increased dramatically since early 2005. But rebel claims that they control a "red corridor" from the Nepalese border in the northeast down to their stronghold in Andhra Pradesh and on to India's southern tip are dismissed by most as exaggerated.

Maoists have a presence in 13 of India's 29 states, but control only tiny pockets within them. There are signs that the conflict may be spreading, however. In 2005, 509 police stations in 11 states reported Maoist-related violence, according to the government. In 2006, some 1,427 police stations in 13 states reported violence, according to The Asian Age newspaper.

In the worst-affected state - Chhattisgarh - tens of thousands of tribal people have been displaced to government camps and, according to a respected human rights centre, there is an "unacknowledged civil war".

The communist movement in India officially began in the 1920s with the formation of the Communist Party of India (CPI).

In 1964 a serious ideological rift within the party, corresponding with the Sino-Soviet split, led a breakaway group to form the Communist Party of India (Marxist) which is now a mainstream Marxist political party.

Some members of the Marxist party were behind a famous uprising in defence of tribal land, in Naxalbari in West Bengal in 1967.

The leaders of the Naxalbari uprising broke away from CPI (Marxist) in 1969 to form the CPI (Marxist-Leninist) - the Maoists, or Naxalites. This party advocated armed revolution and denounced participation in the electoral process.

Soon the Maoists had created vast guerrilla zones stretching from West Bengal to Bihar to Uttar Pradesh and Andhra Pradesh.

But within a few years their fortunes were waning because of internal splits, the death in 1972 of their ideological leader Charu Mazumdar, and major crackdowns by the government.

Since then there have been many communist revolutionary groups operating in different parts of India.

One of the most significant was the People's War Group, formed in 1980 by Kondapally Seetharamaiah, a schoolteacher. It promotes an armed revolution, targeting the state and the security forces as well as oppressive landlords. It began in Andhra Pradesh and spread to Chhattisgarh and Orissa states.

It gained strength in 1998 when it merged with another left-wing group, the Communist Party of India (Marxist Leninist Party Unity).

Partly as a result of such mergers, the Naxalite movement has grown much stronger in recent years.

Naxalites have easily found support among those who feel stranded by India's surging modernisation.

The indigenous people (adivasis), in particular, have historically been subject to discrimination, displacement by large development projects and government failure to ensure food security, say campaigners.

Of the 20 million Indians displaced by development projects between 1960 and 1995, 56% have been tribal people, though they make up only 8.1 percent of the population.

The current programme of globalisation and liberalisation in rural India is thus likely to increase the conflict. In addition, two recent events have fuelled the Maoists' new ascendancy.

Firstly, several internal factions, including the People's War Group, who were previously preoccupied with fighting each other, united in September 2004 to form the Communist Party of India (Maoist).

"We have now organised most of the communist revolutionaries in India," the PWG said at the time.

Secondly, the success of the Maoists in neighbouring Nepal has raised morale and increased manpower, according to Mallika Joseph of the Institute of Peace and Conflict Studies in New Delhi.

In 2005 both countries' Maoists publicly stated their intention to work with each other to spread communism.

Partly because of this cohesion, there has been a dramatic increase in Naxalite violence, particularly in the states of Chhattisgarh, Andhra Pradesh and Jharkhand.

One study, conducted for the Prem Bhatia Memorial Trust, said that by February 2005, Naxalite influence extended over 155 districts in 15 states, affecting close to 300 million people across 7,000 towns.

In November 2005, the authorities were shaken by a Maoist attack on a prison in Jehanabad in Bihar. Some 250 Maoist prisoners were freed, and a paramilitary leader was executed.

In 2006, the Naxalites demonstrated increased capacity for violence. They pulled off some high profile attacks such as the killing of 13 Central Reserve Police Force personnel at Kanjkiro in Jharkhand, in December; and they detained the Tata- Kharagpur passenger train near a deep forest between Gidni and Chakulia stations in Jharkhand in the same month. They have carried out massacres of unarmed civilians that are both "unprecedented" and "chilling", according to the New Delhi-Based Asian Centre for Human Rights (ACHR). In addition, they appear to have increased access to small arms: hauls have included nearly 900 rockets in Andhra Pradesh in September, and over 500 landmines in West Bengal.

Indian police believe there are around 20,000 hardcore Maoist militants, though others put the figure at 10,000.

Prime Minister Singh called a meeting of chief ministers of Naxalite-affected states in April 2006, to develop inter-state coordination in fighting the Naxalites.

They planned an approach that would combine increased security measures with accelerated development.

Later in the the year, the Planning Commission of India said there was a "dismal picture of intra-state imbalance and neglect" across the country. "We cannot let large parts of the country be trapped in a prison of discontent, injustice and frustration that will only breed extremism," it added. "The spread of Naxalism in more than 100 districts in the country is a warning sign."

Its sentiment was echoed by the Ministry of Tribal Affairs which, in a draft revised National Tribal Policy, said: "The solution lies in giving rights to the ST (scheduled tribes) communities over natural and financial resources and addressing the issue of economic deprivation."

But, on the ground, the government response has been weighted far more towards strengthening security than benefiting the impoverished, claims the ACHR.

Military solutions alone cannot succeed because, for one thing, the Maoists wage their war in remote forest regions, says ACHR director Suhas Chakma.

Security analysts also say the rebels have now cleverly hooked onto an issue - the seizure of land by the government for so-called special economic zones (SEZs) - that has angered many poor Indians.

Although many tribal people support the Maoists, they have also suffered brutally at their hands.

The Maoists demand that each family supply one member as a cadre for their ranks. Maoists support themselves by extorting "taxes" from local people. They exact punishment, including execution, on those who resist them.

In Chhattisgarh, the humanitarian situation is particularly bad, and prompted the ACHR to investigate in March 2006.

In one district of this state - Dantewada, an area of 1.1 million people - the government has "outsourced" its war on the Maoists by recruiting villagers to lead the fight.

They operate under the umbrella of a movement known as Salwa Judum, which means "Purification Hunt" or "Peace Campaign" depending on who you ask.

But Salwa Judum, too, has committed atrocities and villagers have been forcibly recruited to the movement.

Villagers now face retribution from either side if they support, or appear to sympathise with the other.

The authorities have evacuated entire villages to makeshift Salwa Judum camps, ostensibly to keep them safe from Maoist attacks. Tens of thousands of tribal people now live there, far from their farms and livelihoods.

The ACHR visited camps in 2006 and 2007, and found conditions "deplorable and sub-human".

It also says both sides have recruited and armed children.

The Salwa Judum campaign has been "disastrous", says the ACHR, leading to a surge in deaths on both sides and among civilians. In Chhattisgarh the situation has become akin to a civil war, it says.

Each Maoist-affected state has a slightly different story.

Andhra Pradesh The government held peace talks with the Maoists in 2004, but these broke down after a few months. Many private armies and state sponsored vigilante groups have sprung up to fight the Maoists.

The government decided in late 2005 to set up a battalion of tribal people to fight the Maoists, pitting tribal people against tribal people.

Bihar There has been a caste war between the Maoists and the Ranvir Sena, a private army of upper caste landlords. Ranvir Sena has killed 1,000 people in the past 15 years. The Maoists have also carried out massacres.

Jharkhand The government has sponsored hunts by villagers for Naxalites and their supporters. Both sides have committed atrocities.

Orissa Local tribes have reportedly been forcibly displaced by the state authorities because they were suspected of sympathising with the Naxalites.

The main states affected by Naxalism are: Andhra Pradesh, Bihar, Chhattisgarh, Jharkhand, Madhya Pradesh, Maharashtra, Orissa, Uttar Pradesh and West Bengal. Other states affected to a lesser extent are Tamil Nadu, Karnataka and Kerala.

The number of displaced is impossible to assess in most states, as villagers quietly escape from afflicted locations to the city or relatives' homes.

"In a country of one billion, no one notices," says Chakma.