Government troops have stepped up security in Chad's capital N'Djamena, as security sources report rebels moving closer to the city.
The country's mobile phone network is cut off, and a VOA reporter in the capital says foreign embassies have advised their citizens to stay at home.
The French Defense Ministry says it will send an additional 150 troops to Chad to supplement the 1,200 French troops already there. French radio said Wednesday that the troops are on standby to help evacuate foreigners if needed.
The apparent offensive by the rebel United Front for Change comes less than a month before presidential elections. The rebels seek the overthrow of Chad's President Idriss Deby.
It is unclear how many rebel troops are advancing toward N'Djamena or how close they are to the capital.
Earlier, Chadian officials said their forces retook control of a town that rebels said they had occupied on Tuesday. The situation in the central town of Mongo remains uncertain.
The rebel movement includes many former government soldiers. Mr. Deby has accused neighboring Sudan of backing the rebels, a charge Khartoum denies.
Mr. Deby came to power in a 1990 coup. Some information for this report was provided by AFP, AP and Reuters.