Fourteen pro-Iran fighters were killed in US air strikes on Syria carried out in retaliation for a drone attack that killed an American and wounded six others, a war monitor said Friday.

A US contractor was killed, and another contractor and five US service personnel wounded, when a kamikaze drone "of Iranian origin" struck a maintenance facility on a base of the US-led coalition near Hasakeh in northeastern Syria, the Pentagon said.

In response, US Defense Secretary Lloyd Austin said Thursday that, at President Joe Biden's direction, he had ordered "precision air strikes tonight in eastern Syria against facilities used by groups affiliated with Iran's Islamic Revolutionary Guard Corps".

"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.

Iran-backed militias have a heavy presence across Syria, especially around the border with Iraq and south and west of the Euphrates in Deir Ezzor province, where the latest US strikes took place.

The Syrian Observatory for Human Rights, a Britain-based monitor with a wide network of sources on the ground, said 14 people had been killed by US strikes, including nine Syrians.

"US strikes targeted a weapons depot inside Deir Ezzor city, killing six pro-Iran fighters, and two other fighters were killed by strikes targeting the desert of Al-Mayadeen, and six others near Albu Kamal," said the Observatory's head Rami Abdel Rahman.

On Friday morning, Iran-backed groups stationed near the city of
Al-Mayadeen fired three missiles near a US base, said Abdel Rahman.

Two missiles struck in Syria's largest oil field, Al-Omar, which houses the US base, without causing damage, while the third landed on a civilian house nearby, he added.

The United States deploys about 900 troops in bases and posts across northeastern Syria as part of the international coalition fighting remnants of the Islamic State group (IS).

- 'Always respond' -

American troops also support the Syrian Democratic Forces (SDF), the Kurds' de facto army in the area, which led the battle that dislodged IS from their last scraps of Syrian territory in 2019.

The US personnel have frequently been targeted in attacks by militia groups.

Two of the US service members wounded on Thursday were treated on site, while the three other troops and one US contractor were medically evacuated to Iraq, the Pentagon said.

"We will always take all necessary measures to defend our people and will always respond at a time and place of our choosing," said General Michael Kurilla, commander of US Central Command.

When the strikes were announced, Biden had already travelled to Canada,where he is set to meet with Prime Minister Justin Trudeau.

In January, the US military said "three one-way attack drones" were launched against the coalition garrison at Al-Tanf in Syria, with one breaching its air defences and wounding two allied Syrian fighters.

The Observatory said it was likely Iran-backed militants had carried out that attack.

Last August, Biden ordered similar retaliatory strikes in Deir Ezzor province after several drones targeted a coalition outpost, without causing any casualties.

That attack came the same day that Iranian state media announced a Revolutionary Guard general had been killed days earlier while "on a mission in Syria as a military adviser".

Iran, a key ally of the government of Syrian President Bashar al-Assad, says it has deployed its forces in Syria at the invitation of Damascus and only as advisers.