Biden blamed Iran-backed groups for the attack, the first deadly strike against U.S. forces since the Israel-Hamas war erupted in October and sent 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.
"Have no doubt - we will hold all those responsible to account at a time and in a manner of our choosing," he said.
Biden later asked for a moment of silence for the three killed service members during a campaign event in South Carolina, adding, "We shall respond."
U.S. Defense Secretary Lloyd Austin echoed that threat. He and other senior officials briefed Biden earlier in the day on the attack.
At least 34 personnel were injured in the attack, but that number is expected to change as more people seek care, according to a statement from U.S. Central Command. Eight personnel were evacuated from Jordan for higher level care, but are in stable condition.
Two U.S. officials said the drone struck near the barracks early in the morning, which could explain the high number of casualties.
The Islamic Resistance in Iraq, an umbrella organization 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.
Since then, U.S. forces have come under attack more than 150 times by Iran-backed groups in Iraq and Syria, causing at least 70 casualties prior to Sunday's attack, most of them traumatic brain injuries. U.S. warships have also been fired at by Iran-backed Houthi forces in Yemen, who are regularly attacking commercial ships passing through Red Sea waters off Yemen's coast.
While the United States has thus far maintained an official line that Washington is not at war in the region, it has been retaliating against the Iran-backed groups in Iraq and Syria and carrying out strikes against Yemen's Houthi military capabilities.
BIDEN PRESSURED ON IRAN
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.
U.S. Senate Republican Leader Mitch McConnell said Biden's inaction was emboldening enemies of the United States in the Middle East.
"The time to start taking this aggression seriously was long before more brave Americans lost their lives," McConnell said.
A senior official with the Iran-backed Palestinian 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, but a person familiar with the matter identified it as Tower 22 in Jordan.
U.S. military activity in Jordan can be a sensitive issue, particularly at a time of heightened tensions due to the Israel-Hamas conflict, because of increasing concern in Jordan about the war spreading.
The Jordanian government said only that the attack occurred at an "advanced post" along its border with Syria.
Late last year, Amman asked Washington to deploy Patriot air defense systems to bolster its border defense.
It has requested more aid to tackle drones used in a raging multibillion-dollar drug war along the border with Syria, which Amman blames on pro-Iranian militias who hold sway in southern Syria.
The kingdom has hundreds of U.S. trainers in the country and holds extensive exercises with U.S. troops throughout the year.
