Thirteen Chadian soldiers were killed in fighting in northern Mali on Friday, the heaviest casualties sustained by French and African troops since the launch of a military campaign against Islamist rebels there six weeks ago, Chad's army said.

Chadian troops killed 65 al Qaeda-linked fighters in the clashes that began before midday in the Adrar des Ifoghas mountains near Mali's northern border with Algeria.

"The provisional toll is ... on the enemy's side, five vehicles destroyed and 65 terrorists killed. We deplore the deaths of 13 of our valiant soldiers," said a statement from the army general staff read on state radio.

France intervened in its former West African colony last month to stop a southward offensive by Islamist rebels who seized control of the north last April.

Troops from neighbouring African nations - including 2,000 soldiers from Chad - have since deployed to Mali and are meant to take over leadership of the operation when French forces begin a planned withdrawal next month.

But continuing violence since the Islamists were driven from major urban areas highlights the risk of French and African forces becoming entangled in a messy guerrilla war as they try to help Mali's weak army counter bombings and armed raids.