Three US service members have been killed by a drone attack on a military base carried out by “radical Iran-backed militant groups” near the Syria border, the White House said on Sunday.
The deaths mark the first time US troops have been killed in an attack in the Middle East since the Israel-Hamas war began in Gaza in October.
President Joe Biden said the US would “hold all those responsible to account at a time and in a manner of our choosing”.
The attack is a significant escalation that is likely to draw the US further into conflict even as Washington seeks to avoid a broader regional conflagration.
The US has in recent months launched several strikes against Iranian-backed Iraqi militants in Iraq and Syria that have launched more than 140 attacks on American forces in the region. But it has not retaliated directly against Iran.
US defence officials said a further 25 service members were injured in the attack at a military base in north-eastern Jordan. One official said the incident happened at the Tower 22 outpost near the Syrian border.
In Iraq and Syria, US forces have come under repeated assault by a newly created group of Iran-backed militias known as the Islamic Resistance of Iraq.
On Sunday, the IRI said it had carried out attacks using armed drones on three military bases housing US troops in Syria, including Al-Tanf. It is not clear if this was the attack that killed the three US service members.
The IRI has said its attacks are in retaliation for Biden’s support of Israel in its war against Hamas in Gaza.
In retaliation, the US has conducted air strikes on facilities linked to the Iraqi militia groups.
Earlier this month, the US military killed a high-ranking commander of Harakat al-Nujaba, an Iran-backed militia in Iraq. Washington described the action as “self-defence” after the faction conducted attacks on US personnel. Experts believe Harakat al-Nujaba is one of the IRI’s most influential factions.
The US and UK have been co-ordinating joint strikes on Houthi targets in Yemen in response to the Iran-backed rebel group’s attacks on vessels sailing through the Red Sea, a critical shipping lane for global trade.
Houthi rebels have said their attacks on shipping lanes were in response to Israel’s bombardment of the Gaza Strip since it launched its war against Hamas in October. The Houthis have carried out more than 30 attacks on international and commercial vessels since mid-November.
Iranian officials have repeatedly said Tehran wants to avoid a wider regional war erupting and did not want to be drawn into direct conflict with Israel or the US.
Earlier on Sunday, chairman of the US joint chiefs of staff General CQ Brown told ABC that he does not believe Iran wants a wider war.
“Iran is probably calculating that the US is reticent to respond and engage in a region-wide conflict; something the administration has been explicit about for months,” said Jonathan Panikoff, a former senior intelligence official now at the Atlantic Council.
He added: “The facts on the ground demonstrate that avoiding the regional conflict is becoming harder regardless of US desires, and the US is now a prime target. That has to prompt the Biden administration to at least reconsider how it views the nature of the current conflict.”
The top Republican on the Senate armed services committee called for direct strikes on Iran.
“We must respond to these repeated attacks by Iran and its proxies by striking directly against Iranian targets and its leadership,” Republican Senator Roger Wicker of Mississippi said.
