Two US service members and an American civilian have been killed in an ambush in Syria, according to the US military's Central Command.
Three others were wounded in the attack and the gunman, a member of the Islamic State group, was killed, it said.
The attack is the first in the country to inflict fatalities on US troops since the fall of president Bashar al-Assad last year.
Pentagon chief spokesman Sean Parnell said the civilian killed was a US interpreter and that the attack targeted soldiers involved in the ongoing counterterrorism operations in the region.
The Britain-based Syrian Observatory for Human Rights, along with local officials, said the attacker was a member of the Syrian security force.
Syria's Interior Ministry spokesman Nour al-Din al-Baba said a gunman linked to IS opened fire at the gate of a military post.
He added that Syrian authorities are looking into whether the gunman was an IS member or only carried its extreme ideology.
He denied reports that suggested that the attacker was a security member.
The shooting took place near historic Palmyra, according to the state-run SANA news agency.
The casualties were taken by helicopter to the al-Tanf garrison near the border with Iraq and Jordan.
US officials respond
US Defence Secretary Pete Hegseth posted on X: "Let it be known, if you target Americans — anywhere in the world — you will spend the rest of your brief, anxious life knowing the United States will hunt you, find you, and ruthlessly kill you."
President Donald Trump told reporters at the White House, "We will retaliate".
He said that Syria's president, Ahmed al-Sharaa, was "devastated by what happened" and stressed that Syria was fighting alongside US troops.
"This is an ISIS attack," Mr Trump said.
Posting on his social media network Truth Social, Mr Trump paid condolences to the three Americans killed and said three others who were wounded "seem to be doing pretty well".
Hundreds of US troops in Syria
The US has hundreds of troops deployed in eastern Syria as part of a coalition fighting IS.
Last month, Syria joined the international coalition fighting against the IS as Damascus improves its relations with Western countries following the ouster of Assad when insurgents captured his seat of power in Damascus.
The US had no diplomatic relations with Syria under Assad, but ties have warmed since the fall of the five-decade Assad family rule.
The interim president, Ahmad al-Sharaa, made a historic visit to Washington last month, where he held talks with Mr Trump.
IS was defeated on the battlefield in Syria in 2019 but the group's sleeper cells still carry out deadly attacks in the country.
The United Nations says the group still has between 5,000 and 7,000 fighters in Syria and Iraq.
US troops deployed in Syria have been targeted in the past, one of the deadliest attacks occurred in 2019 in the northern town of Manbij when a blast killed two US service members and two American civilians.
ABC/wires
