WASHINGTON — The United States military is sharply reducing its emergency casualty evacuation services in West Africa, raising concerns that American troops on missions there could be left vulnerable if they run into trouble at a time when violence is surging in that corner of the continent.

The action by the Pentagon’s Africa Command comes shortly after U.S. military advisers accompanied forces in Niger last month on a major counterterrorism operation near Diffa, a small town on the border with Nigeria that has been a hot spot for attacks by the militant group Boko Haram.

It was the first time American forces in Niger had joined a combat mission alongside their local counterparts since 2017, when U.S. commanders imposed strict guidelines on ground forces. Those new restrictions were imposed after an ambush in October that year near the border with Mali that left four American soldiers dead.

At issue now is the military’s decision to cancel a $23 million annual contract with Erickson, an aviation services company that flies logistics and casualty evacuation flights for Army Green Berets who have been training and advising Nigerien troops for two years in Arlit, a remote city in northern Niger. The Nigerien troops have been conducting operations to intercept terrorists as they flow in and out of Libya.