Three U.S. service members were killed and dozens wounded during an unmanned aerial drone attack on U.S. forces stationed in northeastern Jordan near the Syrian border, President Joe Biden and U.S. officials said on Sunday.

Biden blamed Iran-backed groups for the attack, the first deadly strike against U.S. forces since the Israel-Hamas war erupted in October, sending shock waves throughout the Middle East.

"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.

At least 34 personnel were being evaluated for possible traumatic brain injury, a U.S. official told Reuters, speaking on condition of anonymity.

Two different officials said some wounded U.S. forces were medically evacuated from the base for further treatment.

A fourth official said the drone struck near the barracks, which, if confirmed, could explain the high number of casualties.

The Islamic Resistance in Iraq, an umbrella group of hardline Iran-backed militant groups, claimed attacks on three bases, including one on the Jordan-Syria border.

The attack is a major escalation of the already tense situation in the Middle East, where war broke out in Gaza after Palestinian Islamist group Hamas' attack on Israel on Oct. 7 which killed 1,200.

Israel's subsequent assault on Gaza has killed over 26,000 Palestinians, according to the local health ministry.

While the United States has thus far maintained an official line that Washington is not at war in the region, it has made strikes against targets of Yemen's Houthi groups that have been attacking commercial vessels in the Red Sea.

"We will carry on their 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," Biden said in his statement released by the White House.

Republican opponents of Biden seized on the attack as evidence of the Democratic president's failure to confront Iran as its proxies strike against U.S. forces across the region.

"The only answer to these attacks must be devastating military retaliation against Iran’s terrorist forces ... Anything less will confirm Joe Biden as a coward," said Republican Senator Tom Cotton in a statement.

A senior official with the Iran-backed Palestinian militant group Hamas, Sami Abu Zuhri, directly tied the attack to Israel's campaign in Gaza.

"The killing of three American soldiers is a message to the U.S. administration that unless the killing of innocents in Gaza stops, it must confront the entire nation," he told Reuters.

"The continued American-Zionist aggression on Gaza is capable of exploding the situation in the region."

The U.S. military said the attack occurred at a base in northeastern Jordan, near the Syrian border. It did not name the base. U.S. military activity in Jordan can be a sensitive issue, particularly at a time of heightened tensions of the Israel-Hamas conflict.

A Jordanian government spokesman told state TV that the militants targeted a different base, outside Jordanian territory.

Washington has given Jordan around $1 billion to bolster border security since Syria's civil war began in 2011, and has recently sent more military aid to that end, Western intelligence sources say.

It is unclear which Iran-backed group may have carried out the attack but several U.S. officials said they did not believe that Iranian military forces themselves launched the one-way attack drone.

"While we’re still gathering facts, this is most assuredly the work of an Iranian-backed militia group," another U.S. official said.