The US intelligence community assessed that the one-way attack drone was Iranian in origin, the military said, a conclusion that could further aggravate already strained relations between Washington and Tehran.

Although US forces stationed in Syria have been targeted by drones before, fatalities are extremely rare.

US Defence Secretary Lloyd Austin said the retaliatory strikes were carried out at the direction of President Joe Biden and targeted facilities used by groups affiliated with Iran’s Islamic Revolutionary Guards Corps (IRGC).

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“The air strikes were conducted in response to today’s attack as well as a series of recent attacks against Coalition forces in Syria by groups affiliated with the IRGC,” Austin said in a statement.

“No group will strike our troops with impunity.”

The Syrian Observatory for Human Rights, a group that monitors the war in Syria, said the US strikes had left eight pro-Iranian fighters dead in Syria.

Reuters was unable to independently confirm the toll.

The drone attack on US personnel caused wounds that, for three services members and a contractor, required medical evacuation to Iraq, where the US-led coalition battling the remnants of Islamic State has medical facilities, the Pentagon said. The other two wounded American troops were treated at the base in northeast Syria, it added.

US troops have come under attack by Iranian-backed groups about 78 times since the beginning of 2021, according to US Army General Erik Kurilla, who oversees US troops in the Middle East as the head of Central Command.

Deployments in Iraq, where Iran also holds sway, have also come under drone and rocket attacks in recent years.

Kurilla, testifying to the House Armed Services Committee earlier on Thursday, cautioned about Iran’s fleet of drones.

“The Iranian regime now holds the largest and most capable unmanned aerial vehicle force in the region,” he said.

Three drones targeted a US base in January in Syria’s Al-Tanf region. The US military said two of the drones were shot down while the remaining drone hit the compound, injuring two members of the Syrian Free Army forces.

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US officials believe drone and rocket attacks are being directed by Iran-backed militia, a reminder of the complex geopolitics of Syria where President Bashar al-Assad counts on support from Iran and Russia and sees US troops as occupiers.

The attack came just weeks after the top US general, Mark Milley, visited northeast Syria to assess the mission against Isis and the risk to US personnel.

Asked by reporters travelling with him if he believed the deployment of roughly 900 US troops to Syria was worth the risk, Milley tied the mission to the security of the United States and its allies, saying: “If you think that that’s important, then the answer is ‘Yes’.”

“I happen to think that’s important,” Milley said.

The US deployment, which former president Donald Trump nearly ended in 2018 before softening his withdrawal plans, is a remnant of the larger global war against terrorism that had included once the war in Afghanistan and a far larger US military deployment to Iraq.

While Isis has lost the swathes of Syria and Iraq it ruled over in 2014, sleeper cells still carry out hit-and-run attacks in desolate areas where neither the US-led coalition nor the Syrian army exert full control.

Thousands of other Isis fighters have been detained by the Kurdish-led Syrian Democratic Forces, America’s key ally in the country. American officials say Isis could still regenerate into a major threat.