The US has carried out air strikes against Iran-linked groups in eastern Syria after a drone attack killed a US contractor, the US defence chief said.
Fourteen pro-Iran fighters were killed, according to a monitoring group.
US officials said the air strikes happened on Thursday night, hours after the drone attack, which US intelligence said was “of Iranian origin”.
US bases in north-east Syria have come under attack before, with the US responding with air strikes.
Last August, the US bombed sites in eastern Syria which it said were linked to Iran’s Islamic Revolution Guard Corps (IRGC) – its most powerful armed force – after rocket attacks on US troops there.
Thursday night’s air strikes were carried out on President Biden’s orders “in response to… [the drone] attack as well as a series of recent attacks against coalition forces in Syria by groups affiliated with the IRGC”, US Secretary of Defence Lloyd Austin said.
He said they had targeted “facilities used by groups affiliated with… [the] IRGC”.
Mr Biden, who is in Canada for a working visit, said he was briefed on the death of the US contractor while he was travelling to Ottawa early evening on Thursday.
Speaking to reporters on Friday, Mr Biden offered condolences to the family of the US contractor that was killed, and emphasised that the US does not seek conflict in Iran. But he added: “Be prepared for us to act forcefully to protect our people.”
The conflict continued to escalate on Friday morning when rockets targeted another US base, Green Village, in north-eastern Syria, US officials said. They said the rocket fire did not result in any injuries.
Speaking to CNN on Friday, US National Security Council spokesman John Kirby said the government would “work to protect our people and our facilities as best we can. It’s a dangerous environment,” he said.
Speaking to BBC Newshour, Joel Rayburn, US special envoy to Syria under President Trump, said Iranian-sponsored attacks on US troops in eastern Syria began in 2017.
“It’s usually drone strikes or rockets or mortars, against our bases. Most of the time they don’t do any damage and they don’t cause casualties,” he said.
“The defence secretary’s point is that that’s unacceptable and President Biden authorised a military response against the same kind of guys that carried that out on behalf of the Iranian regime.”
The US defence department said the contractor had been killed, and five US service personnel and a second contractor wounded, when the drone struck a maintenance facility on a coalition base near Hasakah.
The Syrian Observatory for Human Rights (SOHR) – a UK-based monitoring group – said the US strikes killed six pro-Iran fighters in Deir al-Zour city, six in al-Bukamal, and two in the desert of al-Mayadeen.
Iran has not commented.
About 900 US troops are operating out of bases in southern and eastern Syria, without the permission of the Syrian government, as part of the US-led global coalition against the jihadist group Islamic State (IS).
They are tasked with preventing a resurgence of IS, whose militants once controlled large swathes of Syria before being defeated in 2019 following separate campaigns waged by US-backed Kurdish and Arab militia fighters and Iran- and Russia-backed pro-government forces.
The IRGC has built a substantial presence in Syria since the civil war began in 2011, sending hundreds of troops to advise President Bashar al-Assad’s forces and to train and arm thousands of militiamen.
Elsewhere in Syria, SOHR reported that 15 truffle hunters had been killed by IS in the eastern desert.
On Thursday, seven civilians and eight tribesmen working for the Al-Qaterji militia were reportedly killed in Hama, an area where many locals depend on finding truffles for their income.
The fate of 40 other people remains unknown, the observatory said.