Hourmadji Moussa Doumgor, Chad's communication minister, said armed men had attacked Am Timan on Monday afternoon after briefly seizing the town of Goz Beida near the border with Sudan.
France, a former colonial power in Chad, said a ground-to-air missile had been launched against one of its military reconnaissance planes on Monday but missed after the aircraft took "precautionary measures".
The fighters appeared to be moving towards the capital N’Djamena. N'Djamena was attacked in April when rebels approached from Chad's eastern border with Sudan.
Several hundred people were killed in the capital during that attack before the army regained control.
The insurgents, calling themselves the Union of Forces for Democracy and Development (UFDD) - the latest in a string of titles grouping various rebel factions - have said they want elections to end the "catastrophic" rule of Idriss Deby, Chad's president.
Doumgor said: "The security forces are following the movements of these adventurers, whose objective is ... to take advantage of the fact that the towns they have besieged do not have any significant military presence."
The army tightened security in N'Djamena overnight. A witness said there had been troop movements around the streets, while a tank and Jeeps with rocket launchers had taken up position outside the presidential palace.
Bichara Issa Djadallah, defence minister, said: "These are normal movements... We don't need reinforcements from outside, we have sufficient means."
He said he did not believe the rebels would reach the city.
"Last time was a lesson for them. This time they won't risk doing the same thing... Our troops are pursuing them."
A Western diplomat in N'Djamena said the fighters had moved west towards the capital since attacking Am Timan.
"N'Djamena is calm, but there is a rebel presence in the centre of the country... For the time being, there has been no fighting there, but it is probable that the Chadian army will take action," the diplomat said on condition of anonymity.
Deby, who has ruled since seizing power in a 1990 revolt, won elections boycotted by the opposition three weeks after the April assault on N'Djamena.
Deby's government has repeatedly accused the Sudanese government of backing the rebels.
Since the April attack on N'Djamena, the rebel coalition has splintered along rival ethnic and political lines. Recent clashes along the border region with Darfur have involved separate rebel groups rather than one united front.