JOE Biden has vowed to respond after three American troops died following a drone attack in Jordan.
It is the first time US troops have been killed by enemy fire in the Middle East since the beginning of the Gaza war.
The three American troops were killed on Saturday by "radical Iran-backed militant groups" at a small US outpost in northeast Jordan near the border with Syria.
US Central Command confirmed that three service members had been killed.
According to a US official the number of troops injured by the one-way drone attack - which appeared to have come from Syria - was at least 34.
Eight of those injured were evacuated - some in stable but critical conditions, reports CBS.
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President Joe Biden said the US "shall respond" to the drone strike.
He said on Sunday: "Last night, 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.
"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.
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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."
Former president Donald Trump, 77, warned "we are on the brink of World War Three" after the "brazen attack".
There had been more than 158 attacks on US and coalition forces in Iraq and Syria as of Friday - although officials said they rarely caused serious injury or damage to infrastructure.
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.
At this early stage, it is unclear why air defences failed to intercept Saturday's drone attack on Tower 22.
The attack is thought to be the first on Tower 22 since October 17.
US forces are located at the base as part of an advise-and-assist mission with Jordan.
The killing of the three Americans is a significant escalation of the current situation in the Middle East.
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.
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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".
"There is a conflict between U.S. forces and resistance groups in the region, which reciprocate retaliatory attacks," it added.