Palmyra -- President Donald Trump vowed retaliation Saturday after two U.S. Army soldiers and a civilian U.S. interpreter were killed in an attack in Syria, an incident the Pentagon said occurred during a counterterrorism engagement.

"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," Trump said on Truth Social. "There will be very serious retaliation."

Three other U.S. personnel were wounded in the attack in Palmyra, Sean Parnell, the Pentagon’s chief spokesperson said on X. The attack occurred as the soldiers "were conducting a key leader engagement" in "support of ongoing counter-ISIS operations," he said.

"This attack is currently under active investigation," Parnell said.

U.S. Central Command said the attack was an ambush by "a lone ISIS gunman" and that the gunman was engaged and killed.

A Pentagon official told NBC News the attack occurred in an area outside the control of Syria’s interim president, Ahmed al-Sharaa.

Defense Secretary Pete Hegseth said in a separate X post that the person who carried out the attack was killed by U.S. partner forces. He did not provide additional details about the perpetrator of the forces involved.

"The savage who perpetrated this attack was killed by partner forces," Hegseth wrote, adding that those who target Americans "will spend the rest of your brief, anxious life knowing the United States will hunt you, find you, and ruthlessly kill you."

A spokesperson for Syria’s Interior Ministry, Nour al-Din al-Baba, said on state television that internal security officials had issued prior warnings to partner forces operating in the Badia region about the possibility of an Islamic State attack. He said the U.S.-led coalition did not take those warnings seriously.

The U.K.-based Syrian Observatory for Human Rights had reported that the attacker is a member of Syrian security forces. Al-Baba said the individual had no leadership role within Syria's internal security forces and was not serving as an escort to leadership at the time of the attack.

The Department of Defense has not released additional details beyond statements on social media.

President Donald Trump addressed the attack ahead of Saturday's Army-Navy football game in Baltimore, calling it an ambush and mourning the deaths. When asked by a reporter whether the U.S. would retaliate, Trump said, "Yeah, we will."

The U.S. military has about 2,000 troops in Syria, mainly stationed in the northeast, where they work with local security to prevent the resurgence of the Islamic State. The U.S. and other nations have re-engaged with Damascus under al-Sharaa, since rebels ousted former President Bashar al-Assad.