"Our security forces were surprised to come into direct contact with Sudanese armed forces deployed to protect the rebels' rear," he said.

"Chad regrets that there were collateral victims, and by that I mean civilian, in this operation to secure its borders."

But he warned Sudan against any further military action; "If Sudan opts for direct aggression, Chad will use all means to respond."

The Chadian government had earlier said that the army had remained with its borders after routing an attack by the Chadian National Concord on army positions in the border village of Aldjirema on Monday.

Non-aggression pact

Idriss Deby, the Chadian president, and Omar Hassan al-Bashir, his Sudanese counterpart, signed a non-aggression pact in February to try to calm growing tensions between the two countries.

Ali al-Sadig, Sudan's foreign ministry spokesman, 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"

Ali al-Sadig, Sudan's foreign ministry spokesman
"We want peaceful relations but our army will remain vigilant to prevent such actions. Chad is clearly escalating problems," he said.

Sudan's state news agency said that Chad's ambassador had been summoned to the foreign ministry over "his country's attacks on the Sudanese army". 

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 consider sending a peacekeeping force for the east of the country.

The violence came ahead of a visit to Sudan by Thabo Mbeki, the South African president. He is trying to convince Khartoum to accept a plan to deploy UN peacekeepers in Darfur. 
   
N'Djamena accuses Sudan of supporting Chadian anti-government forces based in Darfur, while the Sudanese Arab Janjawid militia are raiding ever further into eastern Chad.

Khartoum, in turn, accuses Chad of supporting groups fighting against the government inside Sudan.

Janjawid attack

Meanwhile, the United Nations said up to 400 people were killed when the Janjawid militia launched an attack into Chad 10 days ago.

"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," Ron Redmond, UN High Commissioner for Refugees spokesman, said.

The UN team found decomposing bodies in the villages, where hundreds of homes had been burnt to the ground, and an "overwhelming stench" from rotting animal carcasses.

Many people who survived the initial attack died from exhaustion and dehydration while fleeing, Redmond said.

One UN official who visited the area described the scene as "apocalyptic".