Chad is formally accusing neighboring Sudan of backing the rebels behind Sunday's incursions in three eastern villages. In the capital N'Djamena, tanks and troops have been deployed to defend the city.
Chad's foreign minister, Ahmat Allam-mi, says a failed surface-to-air missile attack by the rebels on a French reconnaissance plane Monday is proof Khartoum is providing arms to rebels in Chad's east.
The rebel coalition Union of Forces for Democracy and Development denies it has received arms from Sudan. It says it took the so-called Redeye missile from government army stocks. Many of the rebels are high-ranking Chadian army defectors.
Rebels say they are massing 600 kilometers east of the capital N'Djamena. Local sources say troops and tanks have been deployed in the capital to defend the city.
Local journalist Evariste Ngaralbaye says tanks are guarding the city's eastern edge and troops have been seen criss-crossing the city. Ngaralbaye says the scene is reminiscent of last April when rebels reached the outskirts of the capital.
Chad and Sudan signed a peace agreement four months ago and the two countries restored diplomatic relations. Both governments have long traded accusations that each country supports rebel movements in the other.
The Sudanese have claimed Chad is backing rebels in the Darfur-region. N'Djamena, in turn, charges that Sudanese-backed Janjaweed militias, accused of atrocities in Darfur, have crossed the border to attack villages and support rebel opponents of Chadian President Idriss Deby. Sudan has denied this.
Mr. Deby came to power in a coup 16 years ago. Opposition groups boycotted elections last April, just after the rebel assault on the capital.