Three American service members have been killed and dozens more wounded in a drone attack on troops in the Middle East, the US military has said.
US President Joe Biden blamed Iran-backed groups for the attack by an unmanned aerial drone, which he said took place in Jordan near the Syrian border on Sunday.
He said the US "shall respond" to the incident.
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, and we shall respond," he said.With an increasing the risk of military escalation in the region, US officials were working to identify the precise group responsible for the attack, but they have assessed that one of several Iranian-backed groups was behind it.Mr Biden said in a written statement that the United States "will hold all those responsible to account at a time and in a manner (of) our choosing."
US Defence Secretary Lloyd Austin said "we will take all necessary actions to defend the United States, our troops, and our interests."
At least 34 personnel were being evaluated for possible traumatic brain injury, a US official told news agency Reuters.
The Islamic Resistance in Iraq, an umbrella organisation of hardline Iran-backed militant groups, claimed attacks on three bases, including one on the Jordan-Syria border.
The deaths mark the first US fatalities in months of strikes against American forces across the Middle East by Iranian-backed militias, amid the Israel-Hamas war in Gaza - heightening concerns over escalation.
Iran-backed fighters in east Syria began evacuating their posts, fearing US airstrikes, Omar Abu Layla, a Europe-based activist who heads the Deir Ezzor 24 media outlet, told the Associated Press news agency.
According to a US official, the number of troops wounded in the drone attack may grow.
The official said a large drone struck the base, which two other American officials identified as an installation on the Syrian border in Jordan known as Tower 22.
The small installation, which Jordan does not publicly disclose, is understood to include 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.
The conflicting information could not be immediately reconciled.
US troops long have used Jordan - a kingdom bordering Iraq, Israel, the Palestinian territory of the West Bank, Saudi Arabia and Syria - as a basing point in the Middle East.
There are typically around 3,000 American troops stationed in Jordan, according to the Associated Press.
Since Israel's war on Hamas in the Gaza Strip began, US troops in Iraq and Syria have faced drone and missile attacks on their bases. The attack on Jordan marks the first targeting American troops in Jordan during the war.
Mr Biden was in Columbia, South Carolina, but was briefed on the attack on Sunday morning by Defense Secretary Lloyd Austin, national security adviser Jake Sullivan and principal deputy national security adviser Jon Finer.
He said in his statement the US "will carry on their commitment to fight terrorism".
