A gunman who ambushed U.S. and Syrian soldiers, killing two American troops and a civilian interpreter near Palmyra Saturday, is thought to have been an Islamic State infiltrator working as part of local security.

Three other service members were injured in the attack, during which the gunman was "engaged and killed", according to the US military.

Two Army soldiers and a civilian interpreter were killed by a suspected Islamic State attacker who targeted a convoy of U.S. and Syrian forces.