President Donald Trump vows "very serious retaliation" after an ISIS gunman killed two U.S. Army soldiers and an interpreter in a Syrian ambush, leaving three other soldiers wounded.

The ISIS fighter who killed three Americans in Syria over weekend highlighted the continuous U.S. military presence in the country.

Syria's government said it arrested five people on Sunday following the attack that killed three Americans and wounded several others, but it has offered no clarity about how the gunman was able to approach what should have been a secure meeting.

The shooting attack in Palmyra, Syria, on Saturday struck Iowa National Guard troops. Two were killed, along with an American civilian interpreter; three others were wounded.

The shooting happened during a mission by U.S. and Syrian forces in a historic central town near Palmyra on Saturday. President Trump and U.S. military officials attributed the attack to ISIS.

Syria and Iraq held talks on Monday to explore ways to improve border crossing operations and further strengthen cooperation between

The gunman who killed two U.S. soldiers and an American civilian interpreter had been set to be dismissed from the security forces over his extremist views, U.S. and Syrian officials said.

President Trump said there would be “very serious retaliation” for the attack, which also killed an American interpreter.

Sgt. William Nathaniel Howard, 29, and Sgt. Edgar Brian Torres-Tovar, 25, were killed by a "lone ISIS gunman," according to the U.S. military.

The attack on U.S. troops in Syria is the first to inflict casualties since the fall of President Bashar Assad a year ago.