After obtaining United Nations Security Council backing for Deby's government, French President Nicolas Sarkozy warned the rebels France would "do its duty" and had the means to respond to any unlawful attack against its former colony.

Chad has accused Sudan of supporting an offensive from the east by the rebels, who stormed into the capital of the oil-producing central African country at the weekend before withdrawing. Khartoum denies backing the rebels.

Rebel leaders accused France's military of fighting in support of Deby. France used planes and troops based in Chad to evacuate hundreds of foreign nationals from the capital N'Djamena as the fighting raged.

The French military quickly denied this, along with rebel accusations that they had caused civilian casualties.

Deby's government says it beat back and routed the rebels after the weekend's chaotic fighting which left bodies strewn in the dusty streets and hundreds injured. The rebels have said they withdrew to regroup and await reinforcements.

Rebel leaders offered to accept a ceasefire -- but on the condition Deby stepped down.

"Deby must be moved aside," rebel spokesman Henchi Ordjo said. He also accused French military helicopters and tanks of opening fire on Monday in support of Deby's forces in fighting near N'Djamena's airport.

"France has involved itself directly in the conflict ... they've caused civilian victims," Ordjo said.

French armed forces spokesman Christophe Prazuck denied this, but said French forces in the city and at the airport had been caught up in "skirmishes and they fired back. It was legitimate defence," he added.

France lobbied for and obtained a non-binding statement from the U.N. Security Council on Monday, which urged countries to support Deby's government against the rebels.

French President Sarkozy said this meant his country could intervene in Chad in support of Deby if need be.

"If France has to do its duty, it will. Noone should doubt that," he told reporters during a visit to western France.