The spectre of a direct US-Iranian military conflict drew closer on Sunday when the US president Joe Biden announced three US servicemen have been killed and more than 34 injured following a drone attack on a US service base on the border of Jordan and Syria. Biden blamed Iranian backed militia mainly based in Iraq for the “despicable” attack and vowed revenge. Responsibility for Saturday’s attack on Tower 22, a military outpost on the Jordanian Syrian Iraqi borders was claimed by the Iranian backed umbrella group Islamic Resistance, and the US made no attempt to disguise its belief that Iran was ultimately responsible. Four separate drone strikes had been fired at three US bases, and the US was investigating why the T-22 base’s defence mechanism did not repel the drone. Many of the American servicemen wounded have suffered traumatic brain injury, but the extent of injuries has not been disclosed. An official said the drone struck near the barracks early in the morning, which would explain the high number of casualties. US forces have faced a near-daily barrage of drone and missile strikes in Iraq and Syria since the 7 October attack on Israel by Hamas, but this incident draws the US much closer to a direct conflict with Iran, an outcome both sides insist they wish to avoid, but may now be unable to prevent as the incidents proliferate and escalate in impact. It is the first time American military personnel have been killed by hostile fire in the Middle East since the start of the Israel-Hamas war on 7 October, although two US Navy Seals drowned on 11 January off the coast of Somalia as they intercepted a Dhow carrying Iranian weapons bound for Houthi rebels in Yemen. The Islamic Resistance in Iraq, an umbrella group that claimed responsibility for the deaths at T-22, includes Kata’ib Hezbollah group, which fought against coalition forces in Iraq. The Iranian backed groups have long been trying to drive the US troops out of Iraq and Syria, but have used the war in Gaza as the backdrop to intensify these efforts and broaden the battleground. The US says its 900 troops in Syria are working alongside Kurdish Syrian Democratic Forces to defeat Islamic State, the extremist Sunni group. It has about 4,000 troops in Jordan. In a statement Biden pointedly said the US will hold all those responsible to account at a time of the US choosing, and the US Pentagon made no attempt to disguise its belief that Iran is ultimately behind the attacks. Breaking the news, Biden said: “Today, America’s heart is heavy. Last night, three US service members were killed – and many wounded–during an unmanned aerial drone attack on our forces stationed in north-east Jordan near the Syria border. He added: “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. He vowed: “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”. David Cameron, the UK foreign secretary, said Westminster “strongly condemns attacks by Iran-aligned militia groups against US forces”, adding: “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.” Islamic Resistance released a statement saying, “As we said before, if the US keeps supporting Israel, there will be escalations. All the US interests in the region are legitimate targets and we don’t care about US threats to respond, we know the direction we are taking and martyrdom is our prize.” Charles Lister, senior fellow at the Middle East Institute and long-time Syria expert said: “it is a huge escalation and what everyone has been worrying about”. He added “if there is not a truly decisive response to this, the Islamic Revolutionary Guards Corps will feel wholly emboldened. This is the 180th attack since Oct 18 – it must be responded to as the game-changer that it is.” Jordan initially denied the attack occurred on its soil, and later said it took place on the border, in an indication that it does not want to become embroiled in any coming conflict. In a statement, the country condemned the “terrorist attack”, while a senior Jordanian security source told Reuters it had previously appealed to the US for air defence systems and technology to tackle drones. Washington has given Jordan around $1bn to bolster border security since Syria’s civil war began in 2011, and has recently sent more military aid to that end. In a previously recorded interview with ABC News that aired Sunday morning, the Chairman of the Joint Chiefs of Staff Gen CQ Brown said part of the US’s work is to “make sure as things have happened in the Middle East is not to have the conflict broaden”. “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,” he said. Republican opponents of Biden seized on the attack as evidence of the Democratic president’s failure to confront Iran as its proxies strike against US forces across the region. “The only answer to these attacks must be devastating military retaliation against Iran’s terrorist forces … Anything less will confirm Joe Biden as a coward,” said Republican senator Tom Cotton in a statement. Mitch McConnell, the Senate minority leader, called on Biden to “exercise American strength to compel Iran to change its behaviour”; Florida senator Rick Scott said Iran was “blatantly questioning US strength and resolve”. Democrats also joined the calls for action. “Every single malignant actor responsible must be held accountable,” Hakeem Jeffries, the House minority leader, said. A senior official with the Iran-backed Palestinian militant group Hamas, Sami Abu Zuhri, directly tied the attack to Israel’s campaign in Gaza. “The killing of three American soldiers is a message to the US administration that unless the killing of innocents in Gaza stops, it must confront the entire nation,” he told Reuters. “The continued American-Zionist aggression on Gaza is capable of exploding the situation in the region.”