Johannesburg, South Africa ? An attempted coup by rebels in Chad, one of Africa’s poorest and most corrupt nations, could threaten the aid flow to hundreds of thousands of people displaced by the war in the nearby Darfur region of Sudan, thus triggering a massive humanitarian crisis, relief workers warned Sunday.

Chadian troops were using tanks and helicopter gunships Sunday to try to dislodge some 1,500 rebels who poured into the capital, N’Djamena, over the weekend to topple unpopular President Idriss Deby, news reports said.

The rebels, who accuse Deby of being corrupt and favoring his clansmen, are believed by analysts to have attacked from bases inside Sudan, which, like Chad, has long meddled in the affairs of its neighbors. Sudan has denied any role in the fighting. The rebels claimed in phone calls with reporters to have surrounded the presidential palace.

While the fighting grinds on, aid experts worry that the coup attempt could fan a wave of anarchy and looting in the already volatile eastern region of Chad, where some 240,000 Sudanese and more than 100,000 Chadian villagers are languishing in refugee camps after fleeing the civil war in Darfur and associated violence along the lawless Sudan-Chad border.

A $300 million United Nations project to feed and house those refugees is one of the largest aid efforts in Africa.

The U.N. says it has evacuated more than 50 international staff from the east since Friday.

About 200 Chadian staff members remain on duty to distribute some 10,000 tons of food that has been stockpiled to feed the refugees through February.

“The most frightening thing would be not being able to get the food out to the camps,” said Stephanie Savariaud, a spokeswoman for the U.N. World Food program in Dakar, Senegal, the agency’s regional headquarters. “If things start to destabilize, our movements would be limited. We are watching the situation very closely.”

Outside of the capital city of N’Djamena, government troops and rebels have battled only in the eastern border town of Adre, Savariaud said.

The U.N. has dispatched disaster relief experts to neighboring Cameroon, she said, in case thousands of refugees surge across that border from N’Djamena, as they did during a similar rebellion in 1990. So far, about 400 Chadians have crossed the Cameroon border.

Chad has endured chronic coups and revolts since it gained independence from France in 1960.

Deby, a fighter-pilot-turned-president, toppled his predecessor by launching his own invasion from a rebel base in Sudan. Analysts say Sudan is backing the current crop of rebels – who include Deby’s own nephew – in a familiar pattern of tit-for-tat skulduggery, because of Deby’s well-known support for Sudanese rebels in Darfur.

Experts worry that if Deby is deposed, it will be the masses of refugees in the east who will suffer most.

“Deby has used the Darfur rebels as his border guards for years,” said Olivier Bercault, a Chad expert with the New York-based humanitarian group Human Rights Watch. “If Deby falls, those militias will have to retreat into Sudan.

“Then the refugees will be exposed to their enemies in Sudan,” Bercault said. “They will pay the price.”