Abeche, about 160km from the border with Sudan, is the base for humanitarian organisations operating in the east of the landlocked country.
The east, where the UN refugee agency runs camps for thousands leaving Sudan's Darfur region and for displaced Chadians, has seen a number of armed attacks in recent months. The Chad government says some of these offensives are backed by Sudan.
The last offensive in late October by the Union of Forces for Democracy and Development (UFDD), an anti-Deby coalition, was pushed back by government forces. Khartoum denies backing the rebels.
Chad's government on Friday extended for six months a state of emergency in N'Djamena and other regions, including the east, which had been imposed on November 13 to tackle recent ethnic clashes and armed raids that have killed hundreds of villagers.