Chad accused neighboring Sudan of supporting rebels that have attacked three eastern towns in recent days, a claim likely to raise tensions only two months after the countries resumed diplomatic relations.

Foreign Minister Ahmat Allam-mi said the rebels entered Chad from Sudan's western Darfur region in a convoy of 60 to 80 military vehicles and attacked three towns since Sunday, briefly occupying one of them on Monday.

The rebels voluntarily left Am Timan later in the day, said government spokesman Hourmadji Moussa Doumgor. It wasn't immediately clear why the rebels left.

Allam-mi said he summoned Sudan's ambassador in the Chadian capital of N'Djamena to protest Sudan's alleged support for the rebels and Chad's ambassador to the African Union sent a letter of complaint to the body's Peace and Security Council.

"They are well equipped and well armed, even with surface-to-air missiles, which are from Sudanese military stocks," Allam-mi said. "Another group of rebels in (Sudan's) Darfur is preparing to attack army positions in eastern Chad."

Allam-mi said that the second group may attack the border town of Adre imminently. He did not say if the first group has retreated to its bases in Sudan.

Aid workers told The Associated Press on Monday that large numbers of army troops were heading east to intercept the rebels. The rebel force is believed to number less than those who launched a failed attack on the capital in April, the aid workers said on condition of anonymity because it related to security issues.

Following that attack, President Idriss Deby accused Sudan of supporting the Chadian rebels, which Sudan denied. He closed off the border and severed diplomatic ties. The two countries resumed their diplomatic ties and reopened their border in August, four months later.

Alpha Oumar Konare, the chairman of the African Union Commission, said in a statement late Tuesday that he was monitoring developments and in contact with diplomats to examine how best to address the situation. Konare said he was "gravely concerned over the attack perpetrated against Chad by armed elements."

Human rights groups have long warned the ongoing violence in Darfur could destabilize the entire region. More than 200,000 people are believed to have been killed and 2.5 million people displaced in a three-year conflict between Darfurian rebels and the government.

Chadian rebels, who include army deserters and some of Deby's relatives, have had sporadic clashes with the army since October 2005. Deby, who first took power at the head of his own rebel army in 1990, won elections in May that the main opposition parties boycotted because they claimed they had been rigged.

The competition for power in Chad has become more intense since the country began exporting oil in 2004.