Two US Army soldiers and a civilian interpreter were killed by a suspected ISIS attacker, prompting President Donald Trump to vow 'very serious retaliation'.

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.

The two servicemembers killed were members of the Iowa National Guard, according to a person briefed on the situation who was granted anonymity because they did not have permission to speak publicly.