Iowa National Guard soldiers killed in ISIS ambush attack in Syria as Trump vows "very serious retaliation" against the terrorist group in response.

The U.S. troops who were killed and injured in Saturday’s ambush attack in Syria were members of the Iowa National Guard, a senior U.S. official told Fox News.

An ISIS gunman opened fire with an automatic weapon in the central Syrian town of Palmyra, killing two Iowa National Guard soldiers and an American civilian translator, the official said. 

Three additional U.S. soldiers were wounded in the attack.

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Preliminary information suggests the attacker was once affiliated with the Syrian government forces and had been a low-level member of the Syrian Security Forces, before later becoming connected to ISIS, the official said. 

"He was not a part of the official delegation that was being escorted by the U.S. and Syrian troops," the official said. "… This would not be considered a ‘green on blue’ incident because the Syrian gunman was not part of the U.S./Syrian partnered delegation."

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Earlier Saturday, U.S. Central Command confirmed the deaths and injuries were a "result of an ambush by a lone ISIS gunman in Syria."

President Donald Trump warned that there will be "very serious retaliation" following the ambush.

"We mourn the loss of three Great American Patriots in Syria, two soldiers, and one Civilian Interpreter. Likewise, we pray for the three injured soldiers who, it has just been confirmed, are doing well," Trump wrote on Truth Social. 

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Trump added, "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."

Last month, two West Virginia National Guard members were ambushed and shot near the White House in Washington, D.C., in a targeted attack that left one soldier dead.

Fox News' Greg Norman contributed to this report.