The attack happened on Saturday at Sossal on the Chadian border where the deserters were supposed to meet and turn themselves over to Chad's army as well as surrender their arms, the Union of Forces for Democracy and Development (UFDD) said in a statement.

The UFDD rebel group said instead of the army they were met by the Toros-Boros, Sudanese rebels, "who opened fire."

AFP was able to confirm the fighting with local people in the area, but it was not possible Monday to independently confirm the identity of the assailants.

The Chadian government has not commented on the incident.

Several Chadian rebel groups are based in the Sahel desert region of southern Libya.

On Sunday, Chad closed its border with Libya until further notice.

In 2008, the UFDD, with other rebel groups, launched an offensive on the capital N'Djamena to seize power, but they were stopped at the presidential palace by the army with the support of ally and former colonial power France.

In January, another rebel group, the UFR entered northeast Chad from Libya but were stopped by several French air strikes and the Chadian army. French jets intervened at the government's request, France says.

Chad, a vast and mostly desert country with more than 200 ethnic groups, has suffered repeated coups and crises since it gained independence from France in 1960.

Under President Idriss Deby, a former head of the armed forces, the country has taken a leading role in the fight against jihadism in the Sahel region.