"Six shepherds were killed on Saturday morning by fighters from Islamic State group cells... south of the city of Palmyra," the Syrian Observatory for Human Rights said.

The jihadists "killed the shepherds and stole their livestock", the Britain-based war monitor added.

IS overran large swathes of Syria and Iraq in 2014, proclaiming a "caliphate" and launching a reign of terror.

It lost its last territory in Syria in 2019, but its remnants still carry out deadly attacks, particularly in the vast desert which runs from the outskirts of Damascus all the way to the Iraqi border and beyond.

Most of its attacks in recent years have targeted Kurdish-led fighters or loyalists of the now ousted government of president Bashar al-Assad.

Assad fled into exile as Islamist rebel group Hayat Tahrir al-Sham and its allies entered Damascus on Sunday after a lightning offensive.

The Observatory said it had recorded six IS attacks in the Syrian Desert since Assad's overthrow. They killed 18 civilians and more than 50 Assad troops who had abandoned their posts.

On a regional tour, US Secretary of State Antony Blinken told Turkish officials it was "imperative" to work against any IS resurgence in Syria after Assad's fall, and pledged to work with Iraq to ensure the extremist group "cannot re-emerge".

On Thursday, Erdogan assured Blinken Turkey would never ease up in the fight against IS in Syria, despite its operations against US-backed Kurdish fighters seen as key to containing the extremists.