The U.S. pointed fingers at Iran-backed armed groups after three of its soldiers were killed in a drone attack in northeastern Jordan near the Syrian border late Sunday.

It is the first deadly strike against U.S. forces since Israel launched an indiscriminate war on Gaza in retaliation to the Oct. 7 Hamas incursion and marks a major escalation in tensions that have engulfed 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," President Joe 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.

Iran's mission to the United Nations said in a statement on Monday that Tehran was not involved in the attack.

"Iran had no connection and had nothing to do with the attack on the U.S. base," the mission said in a statement published by the state news agency IRNA.

It added: "There is a conflict between U.S. forces and resistance groups in the region, which reciprocate retaliatory attacks."

Biden 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.

Fears of wider conflict

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 armed groups, claimed attacks on three bases, including one on the Jordan-Syria border.

The attack is likely to fan fears of wider conflict in the Middle East, where war broke out in Gaza after the Oct. 7 Hamas incursion of Israel killed 1,140 people.

Israel's subsequent assault on Gaza, however, 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 before 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 targeting the Iran-backed groups in Iraq and Syria and carrying out strikes against Yemen's Houthi military capabilities.

Biden under pressure

Biden's Republican opponents 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 Sen. 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 resistance 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 Israel's war on Gaza, 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 forces who hold sway in southern Syria.

The kingdom has hundreds of U.S. trainers and holds extensive exercises with U.S. troops throughout the year.