BEIRUT, Lebanon — The death toll from US strikes on Iran-linked groups in Syria following a deadly drone attack has risen to 19, a war monitor said Saturday, as Damascus accused Washington of lying about its "vicious attack".

Further rocket attacks by Iran-backed militias took place late Friday, prompting more strikes by coalition warplanes, the Syrian Observatory for Human Rights reported.

Washington carried out the initial strikes after the Pentagon said a US contractor was killed -- and another contractor and five military personnel wounded -- by a drone "of Iranian origin" that struck a US-led coalition base near the northeastern city of Hasakeh on Thursday.

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

The Syrian government, which is closely allied with Iran, accused the United States of lying about the targets of its air strikes to justify its "act of aggression."

"The American lies about the targeted sites are nothing but a failed attempt to justify this act of aggression and the flagrant violation of Syria's sovereignty," the foreign ministry said.

On Saturday, the Britain-based Observatory, which has a wide network of sources on the ground, said 19 people were killed in the first wave of US strikes: three Syrian soldiers and 16 members of Iran-backed militias, 11 of them Syrians.

Following the strikes, Biden sought to lower the temperature saying the United States "does not seek conflict with Iran, but is prepared to act forcefully to protect our people".

More rocket attacks

Hours after the strikes, 10 rockets were fired at American and coalition forces at the Green Village base in northeast Syria, the US Central Command (CENTCOM) said.

There were no injuries or damage to facilities at the base, but one rocket struck a home around five kilometres (three miles) away, causing minor wounds to two women and two children, CENTCOM added.

Iran-backed militias later Friday targeted a base in the Conoco gas field, prompting retaliatory strikes from coalition warplanes on targets in Deir Ezzor city, the Observatory said.

The war monitor said rocket fire then targeted coalition facilities at the Al-Omar oil field base and in the eastern countryside of Deir Ezzor, "causing material damage".

A "cautious calm" returned to the Deir Ezzor area in the early hours of Saturday morning, the Observatory said.

Militias affiliated with Iran's Revolutionary Guards have a heavy presence across Syria, especially around the border with Iraq, and south of the Euphrates in Deir Ezzor province, where the latest US strikes took place.

The United States has about 900 troops in posts across northeastern Syria to keep pressure on the remnants of the Islamic State group and support the Kurdish-led Syrian Democratic Forces, which control most of the northeast.

The Pentagon said two F-15 fighters launched the retaliatory attack -- which spokesman Pat Ryder said was to protect US personnel.

The strikes "were intended to send a very clear message that we will take the protection of our personnel seriously and that we will respond quickly and decisively if they are threatened," he said.

They were "proportionate and deliberate action intended to limit the risk of escalation to minimise casualties," he said.

US personnel in Syria have frequently been targeted in attacks by militia groups the United States says are backed by Tehran.

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

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

"We know that these groups are sponsored by Iran," Ryder said.

"So Iran certainly plays a role in terms of ensuring that this type of activity doesn't happen," he said.

© Agence France-Presse