Three United Nations peacekeepers were killed and three others seriously injured when their vehicle hit an improvised explosive device in northern Mali on Monday, a U.N. spokesperson said.

Islamist militants, some with links to al Qaeda and Islamic State, have been waging an insurgency in northern Mali for the last decade.

The peacekeepers were on a mine search and detection patrol in the northern commune of Tessalit, in Kidal region, when they were hit, spokesperson Stephane Dujarric told reporters.

“We send our deepest condolences to the families of the peacekeepers, to our colleagues in the mission and we wish a speedy recovery to those who were injured,” Dujarric said.

MINUSMA – the United Nations Multidimensional Integrated Stabilization Mission in Mali – currently has about 12,000 military personnel deployed in the country.

At least 174 peacekeepers have been killed in hostile acts in Mali since the start of the mission in 2013, making it the deadliest U.N. peacekeeping mission in the world.