LONDON — The US has carried out air strikes against Iran-linked groups in eastern Syria after a drone attack killed a US contractor, the US defense chief said.

Eight pro-Iran fighters were killed, according to a monitoring group.

Defense 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".

The US defense 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 earlier in the day.

The Syrian Observatory for Human Rights - a UK-based monitoring group - said eight Iranian-backed militiamen were killed in the air strikes, which it said targeted positions in al-Bukamal and al-Mayadeen in Deir al-Zour province, and Deir al-Zour city.

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. — BBC