BAMAKO, Mali (AP) — Three U.N. peacekeepers from Chad have been killed in northern Mali after a roadside bomb exploded as the men approached on foot and several others were seriously wounded, officials said Monday.

Their deaths come just one day before the U.N. Security Council is holding a meeting on the mission in Mali, which was the United Nations’ most dangerous even before the French military pulled its remaining troops from the country in August.

U.N. spokesman Stephane Dujarric said three other peacekeepers were seriously wounded and are receiving treatment in a U.N. hospital.

“This is obviously the latest incident in what is already a very challenging environment for U.N. peacekeeping,” Dujarric said in New York. “This year already, 12 U.N. peacekeepers were killed in Mali in hostile acts.”

The peacekeepers were on a mine search and detection patrol at the time, Dujarric said. Another official in Mali, who spoke on condition of anonymity as they are not allowed to speak to journalists, said the peacekeepers had dismounted and began to advance on foot with a mine detector when the device exploded.

The explosion Monday took place 10 kilometers (6 miles) from the town of Tessalit in Kidal region. Islamic extremists are known to place improvised explosive devices in the area, the official said.

“The modus operandi is new because we have the impression that the device is not only remotely controlled, but that whoever activated it has visual contact with the peacekeepers,” the official said. “Was this visual contact made through a civilian drone? We don’t know at the moment.”

The attack has not been claimed but bears the hallmarks of armed groups linked to al-Qaida.

A French-led military operation forced the Islamic extremists from power in the towns of northern Mali back in 2013. But the jihadis regrouped in the surrounding desert and have launched scores of attacks on the Malian military and U.N. peacekeepers in the years since.

Security concerns across the country have only deepened since the French military withdrew its troops from Mali. France’s decision to move its forces to neighboring Niger came after relations sharply deteriorated with Mali’s junta leader, Col. Assimi Goita, who seized power in an August 2020 coup.

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Associated Press writer Edith M. Lederer at the United Nations contributed.