US forces said they struck more than 70 IS targets in what President Donald Trump described as "very serious retaliation" for the December 13 attack that killed two US soldiers and a US civilian.

Washington had said a lone IS gunman carried out the attack in central Syria's Palmyra, which is home to UNESCO-listed ancient ruins and was once controlled by jihadist fighters.

It was the first such incident since the overthrow of longtime ruler Bashar al-Assad in December last year, and Syrian authorities said the perpetrator was a security forces member who was due to be fired for his "extremist Islamist ideas".

IS has not claimed the attack.

Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, told AFP that "at least five members of the Islamic State group were killed" in eastern Syria's Deir Ezzor province, including the leader of a cell responsible for drones in the area.

Jordan's military said its air force joined the operation "to prevent extremist organisations from exploiting these areas as launching pads to threaten the security of Syria's neighbours and the region, particularly after terrorist organisation IS reconstituted itself and rebuilt its capacities in southern Syria".

- 'Intense bombardment' -

A Syrian security source told AFP that the US strikes targeted IS cells in Syria's vast Badia desert including in Homs, Deir Ezzor and Raqa provinces, and did not include ground operations.

Most of the targets were in a mountainous area running north of Palmyra including towards Deir Ezzor, the source said, requesting anonymity.

US Central Command (CENTCOM) said in a statement that the United States "struck more than 70 targets at multiple locations across central Syria with fighter jets, attack helicopters and artillery".

"The operation employed more than 100 precision munitions targeting known ISIS infrastructure and weapons sites," CENTCOM said, using an acronym for the Islamic State group.

A Syrian security official, also requesting anonymity, told AFP that "the bombardment was intense" and lasted around five hours.

"The targets were far from population centres," the official said, adding that no displacement of residents had been reported and government forces had not been ordered to deploy to the targeted areas.

Syria's foreign ministry, while not directly commenting on the strikes, said on X that the country was committed to fighting IS and "ensuring that it has no safe havens on Syrian territory, and will continue to intensify military operations against it wherever it poses a threat".

- 'Very serious retaliation' -

Trump said in a post on his Truth Social network that the United States was "inflicting very serious retaliation, just as I promised, on the murderous terrorists responsible" for the Palmyra attack.

CENTCOM said that since the attack, US and allied forces have "conducted 10 operations in Syria and Iraq resulting in the deaths or detention of 23 terrorist operatives", without specifying which groups the militants belonged to.

The US personnel who were targeted were supporting Operation Inherent Resolve, the international effort to combat IS, which seized swathes of Syrian and Iraqi territory in 2014.

IS was territorially defeated in Syria in 2019 but still maintains a presence particularly in the country's vast desert.

US forces are currently deployed in Syria's Kurdish-controlled northeast as well as at Al-Tanf near the border with Jordan.

Jordan played a key role in the US-led coalition against the IS, carrying out strikes and making military bases available, while the country has also been the target of IS attacks.

In December 2014, an aircraft belonging to the Royal Jordanian Air Force crashed in Syria and the pilot was later burned alive after being captured by IS.

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