There has been heavy fighting in eastern Chad along the border with Sudan two days after a state of emergency was declared in the region.
The army has been battling various rebellions for years
Soldiers and fighters of the United Front for Democratic Change, a rebel group which has agreed to disarm, clashed in Goz Beida town.
Some 500 rebels, who were waiting to be integrated into the national army, abandoned their posts last week.
Some 3,000 European peacekeepers are due in the region in the coming weeks.
Their mission is to protect some 200,000 refugees who have fled across the border from the Sudanese region of Darfur.
The ethnic violence in eastern Chad has been inflamed by the four-year conflict in Darfur.
Correspondents say 300 people have been killed so far this month.
The Zaghawa tribe, to which Chad's President Idriss Deby belongs, accuses the Sudanese government of supporting members of the rival Tama tribe.
Hopes of an end to the violence were raised recently when the former leader of the United Front for Democratic Change, Mahamat Nour, a Tama, signed a peace agreement and joined the government as defence minister.
But Mr Nour is now accusing the Zaghawas of starting the fresh round of violence.
An army officer told AFP news agency that the rebel fighters in Goz Beida had refused moves to integrate into the regular army.
Earlier this month, the government signed a deal to enter peace negotiations with four other rebel groups.