BAMAKO (Reuters) - Four United Nations peacekeepers and a Malian soldier were killed and 21 people were wounded in two separate attacks by unknown assailants in Mali on Friday, the U.N. mission there said.

Regional armies, U.N. forces and French and U.S. soldiers are struggling to halt the growing influence of Islamist militants, some with links to al Qaeda and Islamic State, in West Africa’s Sahel region.

Mali’s U.N. mission, MINUSMA, has suffered the highest number of fatalities among current U.N. peacekeeping operations.

“I condemn in the strongest terms this attack that has once again befallen the MINUSMA force as well as the (Malian army),” U.N. mission head Mahamat Saleh Annadif said in a statement.

In the first incident on Friday, three peacekeepers and a Malian soldier were killed when they came under attack during a joint operation in the Menaka region near the border with Niger, an area that has seen a spike in violence over the last year.

Sixteen other peacekeepers and one civilian were also wounded.

Later in the day at around noon (1200 GMT), a MINUSMA convoy in the central Mopti region was the target of what the mission described as a “complex attack” by militants using explosive devices and rocket launchers.

One U.N. soldier was killed and three others were seriously wounded, MINUSMA said in a statement.

The mission did not specify the nationalities of the soldiers killed or wounded in either of the attacks.

A 2013 French-led military intervention drove back militants who had seized control of Mali’s desert north a year earlier, but they have regrouped and launch regular attacks against Malian soldiers, U.N. peacekeepers and civilians.

Islamist groups are now increasingly exploiting the porous borders between Mali and neighboring Niger and Burkina Faso to expand their range of operations, alarming Western powers.

France and the United States both have troops deployed in the West Africa.

A new regional force composed of soldiers from Mali, Mauritania, Niger, Burkina Faso and Chad - the so-called G5 Sahel nations - launched its first operations late last month.