Mali and Niger forces were carrying out a joint operation on Monday when a patrol was attacked by "terrorists" near the northeastern town of Tabankort.

The death toll was earlier put at 30 soldiers and 17 jihadist fighters but revised upwards after the discovery Thursday of the bodies of 13 soldiers.

The bodies were "found by a Malian army patrol" in two separate locations, the source said.

Monday's action was another heavy loss for the army, which lost a hundred soldiers in two jihadist attacks in a month in the autumn.

Northern Mali fell into the hands of jihadists in 2012 before the militants were forced out by a French-led military intervention.

Since then, however, the border regions of Mali, Niger and Burkina Faso have become the scene of repeated clashes with jihadist fighters.

Mali's army has been struggling to contain the Islamist insurgency despite help from African neighbours, MINUSMA, the 13,000-strong UN peacekeeping mission in Mali, and former colonial power in the region France.