Three United States service members were killed and "many" wounded in a drone attack overnight on US forces stationed in northeastern Jordan near the Syrian border, US Central Command said.
President Joe Biden on Sunday blamed Iran-backed groups for the attack and has vowed reprisals.
"While we are still gathering the facts of this attack, we know it was carried out by radical Iran-backed militant groups operating in Syria and Iraq," Biden said in a statement released by the White House.
Wounded soldiers evacuated for further treatment
The US Central Command said at least 25 service members were wounded, but the number might rise.
Two different officials told the Reuters news agency that some wounded soldiers were evacuated from the base for further medical treatment.
Another said the drone struck near the barracks, which, if confirmed, could explain the high number of casualties.
Latest in a series of attacks on US and allied forces
Tensions in the region have been rising since the beginning of the Israel-Hamas war, and the attack could exacerbate the situation.
A senior official with the Palestinian militant group Hamas, Sami Abu Zuhri, directly tied the incident to the war in Gaza.
"The killing of three American soldiers is a message to the US administration that unless the killing of innocents in Gaza stops, it must confront the entire nation," he told Reuters.
The drone fatalities are the latest US military deaths in incidents that indicate escalating tension in the region. It comes days after two US Navy Seals, who went missing during an operation to seize weapons bound for Houthis in Yemen, were declared dead.
The latest attack, however, represented the first time US troops had been killed by hostile fire in the region since the war in Gaza began.
There have been over 150 attacks on US and coalition forces in Iraq and Syria since the war began, the Pentagon said. In response, Washington has carried out retaliatory strikes in both countries.
Many of the attacks on US personnel have been claimed by the Islamic Resistance in Iraq, a loose alliance of armed Iran-linked groups that oppose US support for Israel in the Gaza conflict.
US President Biden voiced resolve on Sunday, saying, "We will carry on their [deceased troops'] commitment to fight terrorism. And have no doubt — we will hold all those responsible to account at a time and in a manner of our choosing."
US Defense Secretary Lloyd Austin said he was "outraged and deeply saddened" by the deaths and injuries to troops.
Dispute about exact location of the attack
Jordanian government spokesman Muhannad Mubaidin told public broadcaster al-Mamlaka that the attack had happened outside of the kingdom, across the Syrian border.
But US officials insisted the attack happened at a base in Jordan, known as Tower 22, near the Syria border.
US troops have long been based in Jordan, with up to 3,000 American soldiers typically stationed there.
