Security has been tightened in the Chad capital, N'Djamena, amid reports that rebels are moving towards the city.
The army has been unable to end the rebellion
A BBC correspondent says that tanks are stationed in key areas, such as outside the presidential palace, and military vehicles are on patrol.
Troops have been recalled to base despite the Muslim holiday of Eid - the biggest festival of the year in Chad.
The rebels began their offensive in the east at the weekend but are now said to be near the central town of Mongo.
The BBC's Stephanie Hancock in Chad says there are reports that the outskirts of the capital are heavily fortified with government troops.
On Monday night, the rebels claimed to have seized the town of Am Timan, some 600km from N'Djamena but they are now reported to be just five hours' drive from the capital.
The government has denied that Am Timan had fallen and urged the capital's residents to stay calm.
"The government appeals to the population to remain calm and to go normally about one's business," said spokesman Hourmadji Moussa Doumgor.
On Sunday, the Union of Forces for Democracy and Development (UFDD) rebel seized the small town of Gos Beida.
Our correspondent says there has also been fierce fighting in the border town of Ade.
In April, the rebels took just four days to reach the capital, which they entered, before being repelled.
Chad says the government of neighbouring Sudan backs the rebels - claims denied by Khartoum.
Sudan in turn accuses Chad of backing rebels in the war-torn Darfur region.
Eastern Chad has a similar ethnic make-up to Darfur, where Arab militias are accused of carrying out a genocide against black Africans.