Joe Biden has said the US “shall respond” after three American troops were killed and dozens more were injured in a drone strike on Saturday night in northeast Jordan near the Syrian border.
The president blamed Iranian-backed militias for the first US fatalities after months of strikes by such groups against American forces across the Middle East.
Mr Biden, who was travelling in South Carolina on Sunday, asked for a moment of silence during an appearance at a Baptist church’s banquet hall.
“We had a tough day last night in the Middle East. We lost three brave souls in an attack on one of our bases,” he said. After the moment of silence, Mr Biden added, “and we shall respond”.
As the risk of escalation in the region continues, US officials are working to conclusively identify the group responsible for the attack, but they have ascertained one of several Iranian-backed groups was behind it.
So far one umbrella group, the Islamic Resistance in Iraq, an Iranian-backed militia operating in Iraq and Syria has claimed responsibility for the “suicide” drone attack.
The group has claimed responsibility for dozens of attacks against bases housing US troops in Iraq and Syria since the Israel-Hamas war began.
Mr Biden has said in a written statement that the US “will hold all those responsible to account at a time and in a manner (of) our choosing”.
Defence secretary Lloyd Austin said “we will take all necessary actions to defend the United States, our troops, and our interests”.
Mr Biden’s rival for the White House this year, Donald Trump, said: “This brazen attack on the United States is yet another horrific and tragic consequence of Joe Biden’s weakness and surrender.”
UK Foreign Secretary David Cameron said: “We strongly condemn attacks by Iran-aligned militia groups against US forces. We continue to urge Iran to de-escalate in the region.
“Our thoughts are with those US personnel who have lost their lives and all those who have sustained injuries, as well as their families.”
Iran-backed fighters in east Syria have begun evacuating their posts, fearing US airstrikes, according to Omar Abu Layla, a Europe-based activist who heads the Deir Ezzor 24 media outlet.
According to a US official, the number of troops wounded in the attack by a one-way attack drone may grow.
The official said a large drone struck the base, identified as an installation known as Tower 22.
What is Tower 22?
Tower 22 holds a strategically important location in Jordan, at the most northeastern point where the country’s borders meet Syria and Iraq.
Little is information about the base is publicly known, but it includes logistics support and there are 350 US Army and Air Force troops stationed on site.
Tower 22 is near al-Tanf garrison, which is located across the border in Syria, and which houses a small number of American troops.
Tanf had been key in the fight against Islamic State and has assumed a role as part of a US strategy to contain Iran’s military build-up in eastern Syria.
Tower 22 is located close enough to American troops at Tanf that it could potentially help support them, while also potentially countering Iran-backed militants in the area and allowing troops to keep an eye on remnants of Islamic State in the region.
It is not currently clear what type of weaponry is kept at the base, what air defences are used, nor what exactly went wrong.
It is along the Syrian border with Jordan and largely used by troops involved in the advise-and-assist mission for Jordanian forces.
The small installation, which Jordan does not publicly disclose, includes US engineering, aviation, logistics and security troops.
The US military base at al-Tanf in Syria is just 12 miles north of Tower 22.
The Jordanian installation provides a critical logistical hub for US forces in Syria, including those at al-Tanf, which is near the intersection of the Iraq, Syria and Jordan borders.
Jordanian state television quoted Muhannad Mubaidin, a government spokesman, as saying the attack happened across the border in Syria.
US troops have long used Jordan, a kingdom bordering Iraq, Israel, the Palestinian territory of the West Bank, Saudi Arabia and Syria, as a basing point.
US Central Command put the total casualties at three killed and 34 injured.
Those killed were sleeping in a tent and some of those injured have been evacuated from the country.
Some 3,000 American troops typically are stationed in Jordan.
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Since the war in Gaza began on 7 October, Iranian-backed militias have struck American military installations in Iraq more than 60 times and in Syria more than 90 times, with a mix of drones, rockets, mortars and ballistic missiles.
The attack on Sunday was the first targeting US troops in Jordan during the Israel-Hamas war and the first to kill Americans.
Scores of US personnel have been wounded, including some with traumatic brain injuries, during the attacks.
The militias have said their strikes are in retaliation for Washington’s support for Israel and that they aim to push US forces out of the region.
The US in recent months has hit targets in Iraq, Syria and Yemen to respond to attacks on American forces in the region and to deter Iranian-backed Houthi rebels from continuing to threaten commercial shipping in the Red Sea.
Syria is still in the midst of a civil war and long has been a launch pad for Iranian-backed forces there, including the Lebanese militia Hezbollah. Iraq has multiple Iranian-backed Shiite militias operating there as well.
Jordan, a staunch Western ally and a crucial power in Jerusalem for its oversight of holy sites there, is suspected of launching airstrikes in Syria to disrupt drug smugglers, including one that killed nine people earlier this month.