Chad's army has launched an offensive against rebels operating along the border with Sudan.
Many soldiers have deserted to rebel ranks
Rebel positions in mountains along the border are being attacked, a government spokesman told the Reuters news agency. Two rebel groups are involved.
Tension has increased in recent months, with Chad and Sudan accusing each other of backing rebel groups.
Last week, the Chad authorities said they had foiled an attempt to stage a coup against President Idriss Deby.
Clashes are reported between government forces and two rebel groups operating in the area, Scud led by Yaya Dillo, and RPJ, led by Abakar Tollimi.
"We lost about 10 people on our side, and at least 25 were injured," Mr Dillo told the BBC's French service.
"On the government side at least 100 people were killed," he said.
Observers in the border area have seen convoys transporting Chadian government forces and weapons, and President Idris Deby is reported to be supervising the fighting.
'State of war'
Analyst Andrew Manley has told the BBC that in recent months President Deby has been looking increasingly vulnerable, faced with the growing rebellion in the east and a loss of support among neighbouring countries and traditional allies like France.
In December, Chad declared a state of war with Sudan following a deadly attack launched from Darfur by Chadian rebels.
Sudan repeatedly denied allegations made by Chad that it was backing the rebels and sending Arab militias in support.
In February, Chad and Sudan signed an accord to resolve their differences over fighting along the border.
Mr Deby seized power in 1990 after launching a rebellion from bases in Darfur.