The 21 were killed in at least 130 air strikes "carried out over the past 24 hours by the Russian air force targeting the Islamic State group" in a vast area stretching from the central province of Homs to the border with Iraq, the Syrian Observatory for Human Rights said.

The raids, which continued into Saturday, follow a series of IS attacks Friday on government and allied forces that killed at least eight members of a pro-Damascus militia, the Britain-based monitor said.

More than half of the slain jihadists were killed in strikes on the eastern province of Deir Ezzor, near the Iraqi frontier, according to the Observatory.

Russian raids in the desert region generally "target small groupings of IS militants as well as their vehicles", said Rami Abdul Rahman, who heads the Observatory.

"It is a difficult operation for the Russians because there are no fixed positions for IS fighters who are always on the move," he told AFP.

In recent months, the vast desert, know in Arabic as the Badia, has been the scene of increasingly frequent fighting between the jihadists and government forces backed by Russian air power.

The region provides a "safe haven" for jihadists planning attacks on government forces and other rivals, the United Nations said in a report published this month.

IS overran large parts of Syria and Iraq and proclaimed a cross-border "caliphate" in 2014, before multiple offensives in the two countries led to its territorial defeat.

The jihadists continue to launch attacks, mostly in the Badia.

IS retains some 10,000 active fighters in Iraq and Syria, although the majority are reported to be in Iraq, the UN says.

Since Syria's civil war broke out in 2011, more than 387,000 people have been killed and millions forced from their homes.