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 an area on the edge of the provinces of Aleppo, Hama and Raqqa", 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.

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.

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 desert which stretches from the central province of Homs to the border with Iraq.

More than 1,300 government troops have been killed in these clashes, as well as 145 pro-Iran militia members and more than 750 IS jihadists, according to the Observatory.

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