By Michael Georgy KHARTOUM, April 10 (Reuters) - Sudan vowed a firm response, including possible military action, to what it said was a Chadian army attack that killed 17 of its soldiers, as the two neighbours traded accusations of cross-border aggression. Chad denied its forces crossed the border but said it had routed a major attack on its territory by a convoy of 200 rebel vehicles launched from Sudan on Monday. It said at least eight Chadian soldiers and numerous rebels were killed. The incident marked a sharp deterioration in volatile relations between the two central African neighbours, marred by violence spilling over from the crisis in Sudan's Darfur region. As the confrontation worsened, the United Nations said up to 400 people had been killed in Chad in cross-border attacks by Sudanese Janjaweed militia 10 days ago and one U.N. official who visited the remote area described scenes as "apocalyptic". Decomposing corpses, hundreds of burnt out houses and animal carcasses were left after the carnage and even more people died, particularly the young and elderly, of exhaustian and dehydration in a mass exodus of terrified residents. Chinese Deputy Foreign Minister Dai Bingguo and U.S. Deputy Secretary of State John Negroponte spoke by phone, intensifying diplomatic efforts to resolve the situation in Darfur. South African President Thabo Mbeki was due in Khartoum later on Tuesday for talks with his Sudanese counterpart Omar Hassan al-Bashir and is expected to discuss Darfur. In the latest clash, Chad said a large rebel force was defeated after crossing from Sudan on Monday in an attempt to destabilise its government, but Khartoum accused Chad's army of crossing into its territory and killing its troops. "Sudan's response will be strong. We will consider all responses, political, diplomatic and military," said a Sudanese army spokesman, who asked not to be named. Chadian Information Minister Hourmadji Moussa Doumgor said a large rebel force from Sudan was defeated after attacking army positions in the border village of Aldjirema. It is just two months since Chadian President Idriss Deby and Bashir signed a non-aggression pact in the Libyan capital Tripoli in a bid to calm rising regional tensions. Sudan's foreign ministry spokesman, Ali al-Sadig, accused Chad of violating the agreement. "We want peaceful relations but our army will remain vigilant to prevent such actions. Chad is clearly escalating problems," he said. FLOODS OF REFUGEES The four-year war in Darfur, which has killed an estimated 200,000 people, has driven hundreds of thousands of refugees into Chad and prompted the United Nations to study a peacekeeping force for the country's lawless east. N'Djamena accuses Sudan of supporting Chadian rebels based in Darfur, while Sudanese Arab militia known as "Janjaweed" are raiding ever further into eastern Chad. The UNHCR refugee agency said last week Chadian authorities reported an initial toll of at least 65 dead in March 31 attacks on two villages in eastern Chad where about 8,000 people lived. But the new estimate of up to 400 followed a visit by a group of U.N. agencies to the area on Sunday. "Estimates of the number of dead have increased substantially and now range between 200 and 400. Because most of the dead were buried where their bodies were found -- often in common graves owing to their numbers -- we may never know their exact number," UNHCR spokesman Ron Redmond told a briefing. The U.N. team found decomposing bodies in the villages, where hundreds of homes had been burned to the ground, and an "overwhelming stench" from rotting carcasses of dead animals. "There were many indications that people had little or no time to flee, given that many essential household goods, food and domestic animals were left behind," Redmond said. Many of those who survived the initial attack died from exhaustion and dehydration often while fleeing, according to the U.N. High Commissioner for Refugees (UNHCR). In their late night diplomatic contact, Negroponte and his Chinese counterpart discussed the peace plan put forward by former U.N chief Kofi Annan to deploy a hybrid African Union-U.N. peacekeeping force in the region. "In their talks, the two sides discussed issues concerning the implementation of the Annan plan," a Chinese foreign ministry official said, adding: "But it requires consultations between the Sudan government, African Union and the United Nations to decide the timing of the deployment and implementation of the plan." Negroponte is expected to visit Sudan and give a tough message from Washington, which has threatened new measures in an effort to break resistance to sending international troops to back the ill-equipped African Union forces. China, which buys much of Sudan's oil and wields veto power over U.N. resolutions, faces criticism from Western powers and rights campaigners for having rejected a U.N. force deployment.
