JOHANNESBURG, South Africa – The war in Sudan’s anarchic region of Darfur spread across a swathe of Africa and engulfed neighboring Chad yesterday when rebels attacked its capital, N’Djamena.

Gunfire echoed across the city and mortar rounds exploded in the streets as heavily-armed insurgents launched their dawn raid. President Deby of Chad, holed up in his palace, ordered his tanks and helicopter gunships into action.

By early afternoon, when the fighting died down, he claimed in a broadcast that he was in “full control” of his capital. Mr. Deby said the “rebel column” had been driven out and stability would soon return.

In fact, his position could scarcely be more precarious. Only last month he survived an attempted coup.

Now the rebels, who reached the center of N’Djamena before being beaten back, have achieved a lightning advance across the vast desert country.

Their offensive began from bases inside Sudan’s war-torn region of Darfur, which shares a frontier with Chad. Western diplomats and Mr. Deby’s regime accuse Sudan of arming these rebels and launching them against Chad.

The latest developments mark an escalation of the conflict in Darfur, which has claimed the lives of 300,000 people and turned 2 million into refugees.

In a matter of weeks the rebels have managed to leave Darfur and advance 600 miles across an arid landscape to reach N’Djamena and threaten Mr. Deby.

As recently as Monday, the rebels, styling themselves the United Forces for Change, were reported to be 250 miles east of the capital. By Wednesday they had managed another leap forward. Yesterday they reached the heart of the city before Mr. Deby drove them back.

Western diplomats believe Sudan is trying to oust Mr. Deby in retaliation for his role in the Darfur war. Khartoum has accused him of arming the rebels who began the fighting in Darfur.

Mr. Deby comes from the black African Zaghawa tribe, also present in Darfur. The Zaghawas were among the tribes who rose up against Khartoum’s control of Darfur.

Sudan believes Mr. Deby sent arms to the main rebel group in Darfur, styling itself the Sudan Liberation Army. Western diplomats have no doubt Sudan responded by arming Arab insurgents inside Chad and dispatching them to overthrow Mr. Deby.

He is deeply unpopular in much of Chad, where Zaghawas make up only 7% of the population. The Arab tribes are his traditional opponents. Mr. Deby, who seized power in 1990, is a close ally of Chad’s former colonial power, France.