Washington said a lone gunman from the IS jihadist group carried out the December 13 attack in Palmyra -- home to UNESCO-listed ancient ruins and once controlled by jihadist fighters -- that killed two US soldiers and a US civilian.

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

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.

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.

Using another acronym for IS, CENTCOM said "the operation employed more than 100 precision munitions targeting known ISIS infrastructure and weapons sites".

President Donald Trump said in a post on his Truth Social network that the United States is "inflicting very serious retaliation, just as I promised, on the murderous terrorists responsible", and that those who attack Americans "WILL BE HIT HARDER THAN YOU HAVE EVER BEEN HIT BEFORE".