“We are now absolutely certain that it is the Janjawid that carried out this incursion, as in the past, for reasons we do not know,” Deby told Radio France Internationale. “What is sure is that they cannot go unpunished.”

Chad has said that armed men in military uniform launched an attack in the eastern Ouaddai province on Monday, killing 36 herders and stealing livestock. Eight insurgents and two Chadian soldiers were killed in an ensuing clash, a government statement said.

Cross-border raids and armed clashes are not unusual in eastern Chad, which abuts Sudan’s volatile Darfur region.

The Darfur conflict - where the Janjawid militia allied with the Sudanese government are pitted against rebels of the same ethnic group as Deby - has the Chadian president in a tight spot.

Deby initially took office in 1990 with Sudan’s backing, but in the past year he has occasionally taken a tougher line against Khartoum, earlier this year accusing his neighbour of backing 3,000 Chadian rebels.

One analyst said that in blaming the Janjawid - and not directly fingering the Sudanese government - Deby appears to want to paint himself as neutral in the long-standing conflict in Darfur and maintain a delicate balancing act.

“This was a nice show of neutrality,” said Roland Marchal, Africa specialist at the Sciences Po university in Paris. “Deby was very prudent not to implicate the government of Sudan. In the end, he is responsible for the country’s relations with Sudan.”

The Chadian president said it was too early to say whether the attackers were backed by the Sudanese government.

"I cannot figure it out. These people were armed to the teeth with weapons and so many munitions,” he said. “Who gave it to them? Is it the government of Sudan, or another? We’ll find out.”

A Chadian government delegation has travelled to the site of the attack, about 1,000 kilometres from the capital, N’djamena, to “take the necessary measures,” according to the government communique.

The raids in Chad came just as the United Nations warned that continuing violence in Darfur itself is hindering humanitarian efforts.

If the violence continued to escalate and it continued to be so dangerous to unarmed humanitarian workers, the UN might not be able to sustain its operations for 2.5 million people requiring life-saving assistance there, officials said.

All this as the sixth round of peace talks carry on in the Nigerian capital, Abuja, between the Sudanese government and the Darfur rebel groups -- who say they are fighting to end the neglect and oppression of the semi-desert region. Progress so far has been slow.

Deby said the Darfur conflict was weighing heavily on impoverished Chad, which has taken in some 200,000 refugees from Darfur, and said it was keen to see an end to the two-and-a-half year crisis. The government estimates the crisis has cost it tens of millions of dollars, as well as posing security issues and damaging the environment.

“Chad is very eager to see the international community, in particular the African Union, settle this crisis.” Deby said.

One obstacle at the talks had been accusations of bias made against Chad, one of a number of mediators, by the Justice and Equality Movement (JEM), one of two rebel groups round the table in Abuja.