The Syrian Observatory for Human Rights, which monitors casualties of the decade-old conflict, said the overnight strikes focused on an area between Palmyra and Al-Sukhna.

"The jihadist organisation's members hide in caves in this area," Observatory chief Rami Abdel Rahman said.

The strikes killed 11 IS fighters and wounded around 20 others, some of them seriously, he said.

Abdel Rahman said he had counted a total of 229 Russian air strikes against targets in the Syrian desert already this year.

Thursday's strikes were the deadliest of their kind since November, when the Observatory reported 16 killed in jihadist ranks.

The Islamic State group's self-declared caliphate once stretched across vast parts of Syria and Iraq and administered millions of inhabitants.

A long and deadly military fightback led by Syrian and Iraqi forces with backing from the United States and other powers eventually defeated the jihadist proto-state in March 2019.

The remnants of IS mostly went back to their desert hideouts from which they continue to harass Syrian government and allied forces.

The group is thought to be attempting to secure sources of funding through trafficking and racketeering, prompting observers to warn of a jihadist resurgence in the region.