Chad's government says its forces are battling insurgents along its border with Sudan. The fighting comes amid mounting threats against the rule of Chadian President Idriss Deby and the risk of Sudan's Darfur conflict spilling into Chad.
Chadian government officials say the fighting near the eastern border town of Adre and its military barracks, left about 100 people dead, but this could not be independently confirmed.
Aid workers in the region said they were treating a dozen wounded.
Chadian officials said they reserved the right to pursue the insurgents into Sudan, in areas where Darfur rebels fighting against Sudan's government are also active.
The rebels behind the attack are believed to include recent defectors from the Chadian army who have called for the overthrow of President Idriss Deby.
They have been accused of other recent attacks on military barracks in Chad, including in the capital N'Djamena, before retreating east. They are believed to be regrouping on the Sudanese side of the border.
An analyst with the London-based group Global Insight, Chris Melville, says this is an extremely tenuous time for Mr. Deby, who took power in a coup 15 years ago.
"The threats to his regime are clearly multiplying at a fairly rapid rate," he said. "The latest wave of defections from the army suggest that the opposition to Deby from within the regime itself appears to be reaching some kind of critical mass."
Mr. Melville says the growing instability comes as Chad's government has had an increasingly difficult time in accommodating hundreds of thousands of refugees who have fled Sudan's Darfur conflict.
"I would not necessarily say that it was the refugee situation in eastern Chad that has created Deby's recent problems, although of course it has contributed to Mr. Deby's financial problems and his apparent inability to fund a serious military counter-offensive against the defectors," Mr. Melville added.
The Sudanese government has complained that some of the refugees in Chad are regrouping into new rebel forces, attacking their government positions along the border.
Mr. Deby has tried to help mediate peace in Darfur and restore security in the area, but his efforts, as well as ongoing attempts by the African Union have not stopped the fighting.