A DEADLY drone attack on a US base near Syria hit the soldiers' living quarters contributing to the high number of casualties, officials said.
Three US troops were killed and another 34 were injured in the attack - marking the first time American troops have been killed by enemy fire in the Middle East since the beginning of the Gaza war.
The troops were killed on Saturday by "radical Iran-backed militant groups" at a small US outpost in northeast Jordan, US officials said.
The Pentagon called the strike "a one-way ‘suicide’ drone attack on a patrol base".
Iran has denied any involvement in the attack - and Jordan says the strike took place in Syria, not inside Jordan.
At this early stage, it's unclear why air defences failed to intercept the drone attack on the outpost known as Tower 22 in Rukban.
US officials said at least 34 military personnel were being evaluated for possible traumatic brain injury - and eight of the wounded were evacuated from the base for treatment.
They said the drone struck the living quarters - which could explain the high number of casualties, according to the .
President Joe Biden said the US will "respond" and "hold all those responsible to account at a time and in a manner of our choosing".
He said on Sunday: "Three US service members were killed - and many wounded - during an unmanned aerial drone attack on our forces stationed in northeast Jordan near the Syria border.
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"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."
The president said that the three American service members killed were "patriots in the highest sense" as he vowed to hold those responsible for their deaths to account.
He added: "We will carry on their commitment to fight terrorism.
"And have no doubt - we will hold all those responsible to account at a time and in a manner our choosing."
The names of the servicemen killed and wounded have not yet been released.
Meanwhile, former president Donald Trump, 77, warned "we are on the brink of World War Three" after the "brazen attack".
In , he said: "The drone attack on a US military installation in Jordan, killing 3 American service members, and wounding many more, marks a horrible day for America.
"My most profound sympathies go to the families of the brave service members we have lost.
"This brazen attack on the United States is yet another horrific and tragic consequence of Joe Biden’s weakness and surrender."
Since the war in Gaza began October 7, 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.
One US soldier was said to have been critically injured in an attack in Iraq on Christmas Day, although the Pentagon classified most of the other injuries sustained by US troops since the attacks began - of which there have been about 70 - as minor.
The attack is thought to be the first on Tower 22 since October 17.
The logistics support base is along the Syrian border and used largely by troops involved in the advise-and-assist mission for Jordanian forces.
The small post - which Jordan does not publicly disclose - includes US engineering, aviation, logistics and security troops.
US Central Command said approximately 350 US Army and Air Force personnel were deployed to the base.
The killing of the three Americans is a significant escalation of the current situation in the Middle East.
Prime Minister Rishi Sunak said: "We are concerned and would urge Iran to continue to de-escalate tensions in the region.
"We absolutely condemn what has happened over the past couple of days.
"My thoughts are with all of those impacted, those who lost their lives, their families and those that are injured.
"We stand resolutely with our allies to bring stability and peace to the region. And that's what we'll continue to work towards."
In a statement on Sunday following the news of the three Americans killed, Sen. Lindsey Graham said the Biden administration’s "policy of deterrence against Iran has failed miserably".
“The Biden Administration can take out all the Iranian proxies they like, but it will not deter Iranian aggression," Graham said.
"I am calling on the Biden Administration to strike targets of significance inside Iran, not only as reprisal for the killing of our forces, but as deterrence against future aggression."
It comes after Chairman of the Joint Chiefs of Staff General Charles Quinton Brown commented on the US' role in the conflict during a previously-recorded interview with ABC News that aired this morning.
He said that part of the US' work is to "make sure as things have happened in the Middle East is not to have the conflict broaden."
Gen. CQ Brown added: "The goal is to deter them and we don’t want to go down a path of greater escalation that drives to a much broader conflict within the region."
Iran's foreign ministry spokesperson Nasser Kanaani said claims that Iran was involved in the attack were "baseless".
Kanaani said "resistance groups" do not take orders from the Islamic Republic.
Earlier, Iran's Mission to the US issued a statement insisting that "Iran had no connection and had nothing to do with the attack on the US base".
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"There is a conflict between US forces and resistance groups in the region, which reciprocate retaliatory attacks," it added.
Iran-backed fighters in east Syria have started evacuating their posts, fearing US. airstrikes, according to Omar Abu Layla, a Europe-based activist who heads the Deir Ezzor 24 media outlet.