Chadian troops have fought rebels for more than a decade
Chadian government forces have launched air strikes and ground assaults against rebels in the east of the country, in the first major fighting since May.
Rebels from the Union of Resistance Forces (UFR) say four civilians were killed and many more wounded in the clashes, near the Sudan border.
But the government denies that troops have caused any civilian casualties.
Rebels almost succeeded in toppling the government last year but were repelled by French troops.
The BBC's Celeste Hicks in N'Djamena says the rebels are much weaker now than they were last year.
She says the skirmishes with government troops are likely to be an attempt by the rebels to show the country that they are still a significant force.
Chad has suffered from rebellions by several armed groups for more than a decade.
The government claims Sudan backs and arms the militants - allegations denied by Khartoum.
Sudan in turn accuses Chad of backing rebels in its troubled Darfur region.