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.

“This was an ISIS attack against the U.S., and Syria, in a very dangerous part of Syria, that is not fully controlled by them,” he said in a social media post.

The American president told reporters at the White House that Syria's president, Ahmed al-Sharaa, was “devastated by what happened” and stressed that Syria was fighting alongside U.S. troops.

U.S. Central Command said three service members were wounded in an ambush Saturday by a lone IS member in central Syria. Trump said the three “seem to be doing pretty well.” The U.S. military said the gunman was killed.

The attack on U.S. troops in Syria was the first with fatalities since the fall of President Bashar Assad a year ago.

The shooting took place near historic Palmyra, according to the state-run SANA news agency, which earlier said two members of Syria’s security force and several U.S. service members had been wounded. The casualties were taken by helicopter to the al-Tanf garrison near the border with Iraq and Jordan.