N’Djamena, Chad ? Government forces used attack helicopters, tanks and heavy weapons Thursday to beat back rebels who charged 600 miles in pickup trucks from the Darfur border to reach the capital of this volatile, oil-producing nation in the center of Africa.

The assault underscored concerns that the civil war in Darfur, part of western Sudan, has undermined Chad President Idriss Deby’s authority and destabilized the entire region.

Deby blamed Sudan’s government for backing the rebels in Chad, where more than 200,000 refugees from Darfur have fled. Sudan denies the claim and accuses Chad of backing a different rebel group in Darfur.

Deby said his soldiers repelled the second attempt to overthrow him in a month, but few in Chad believe the fighting is over. Residents of the outlying neighborhoods who felt the brunt of the attack moved into the center of the capital late in the day, expecting another offensive.

In just three days, the rebel United Front for Change made the journey from their bases on the border with Darfur and came close to capturing the National Assembly building in the capital, N’djamena.

Government troops pushed them back. Deby presented 285 prisoners to the media Thursday evening. Several described how senior army officers had collaborated with the rebels, saying they did not expect to meet any resistance when they entered the capital. Their stories could not be independently verified.