WASHINGTON — A sweeping Pentagon review of elite United States commando missions is likely to result in a sharp cut — by as much as half over the next three years — in Special Operations forces in Africa, military officials said.

Ordered by Defense Secretary Jim Mattis in recent weeks, the assessment of Special Operations units worldwide follows an ambush in Niger that killed four American soldiers last fall. The review is an outgrowth of a Defense Department strategy that focuses on combating rising threats from Russia and China.

More than 7,300 Special Operations troops are working around the world, many of them conducting shadow wars against terrorists in Yemen, Libya, Somalia and other hot spots. The Special Operations Command in Tampa, Fla., has also assumed important new missions in recent years, like taking the lead on combating weapons of mass destruction.

Pentagon officials said Mr. Mattis and Gen. Joseph F. Dunford Jr., the chairman of the Joint Chiefs of Staff, are worried that the commandos are spread too thin. The two leaders have ordered the military’s Special Operations and Africa commands to present a range of options by mid-June to balance rising security challenges — which also include North Korea and Iran — with vital counterterrorism operations.