The Sudanese foreign ministry summoned Chad's Charge d'affairs in Khartoum and handed him a letter warning N'djamena that Khartoum would retaliate for the killing of its soldiers by Chadian forces. Sudan and Chad traded threats and accusations on Tuesday (April 10) over cross-border attacks, cranking up tensions on their already violence-torn common frontier. After talks with the Chadian ambassador, the Sudanese government promised a firm response, including possible military action, to what it said was a Chadian army attack on Monday (April 9) that killed 17 of its soldiers. "Moments ago, the foreign ministry summoned the Chadian Charge d'affairs in Khartoum and informed him about Sudan's strong protest over the violation by Chadian forces of the sanctity of Sudanese territory and the crossing of its forces and clashes with the Sudanese army, which led to the deaths of several officers and soldiers," said Sudan's Foreign Ministry spokesman, Ali al-Sadig. "He (the Chadian Charge d'affairs) was informed that Sudan reserved the right to respond to this serious violation at a time and place that our armed forces shall determine," he added. While Chad denied any deliberate assault on its eastern neighbour, it acknowledged its forces had clashed with Sudanese troops after crossing the border to pursue Sudanese-backed rebels it said were launching raids. The incident marked a sharp flare-up of tension between the two oil-producing central African neighbours, whose ties have been increasingly marred by violence spilling over from the conflict in Sudan's Darfur region. Reversing an earlier denial, Doumgor said the Chadian army had used its "right of pursuit" to chase the rebels into Sudan, where he said Sudanese troops were protecting them. Without acknowledging Sudanese military deaths, he said Chad regretted "civilian victims" had been killed in the fighting. The four-year war in Darfur, which has killed an estimated 200,000 people, has driven several hundred thousand 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. In the latest border clash, Chad said a large rebel force was defeated after crossing from Sudan on Monday in an attempt to destabilise its government. It is just two months since Chadian President Idriss Deby and Bashir signed a non-aggression pact in the Libyan capital Tripoli to try to calm rising regional tensions.