President Donald Trump says “we will retaliate” after two U.S. service members and one American civilian were killed in an attack in Syria that the United States blames on the Islamic State group.

An assailant opened fire at Syrian and U.S. forces on Saturday during a visit by American troops to a historic central town, leaving several wounded, Syria’s state media and a war monitor said.

Two U.S. service members and one American civilian were killed and three other people wounded in an ambush on Saturday by a lone member of the Islamic State group in central Syria, the the U.S. milita...