At least 38 Islamic State group fighters were killed in air strikes by the Damascus regime against three jihadist-held towns in central Syria, a monitoring group said on Tuesday.

Monday's strikes hit Palmyra and two other towns in Homs province, Syrian Observatory for Human Rights director Rami Abdel Rahman told Agence France Presse.

The Syrian air force has been increasing its strikes against IS in recent days as it received reinforcements from Russia, he said.

"The number of raids is growing and the strikes are more precise after the Syrian air force received arms and more efficient planes from Moscow," said Abdel Rahman, whose group relies on broad network of civilian, military and medical sources inside Syria.

Experts close to the regime have told AFP that Russia has sent advisers to train Syrian troops in the new weapons, in particular short-range air defence systems and tanks.

U.S. military officials told AFP on Monday that Russia had deployed 28 combat planes in Syria, in the latest move in Moscow's increasing military presence in the war-torn country.

Washington in recent weeks has expressed growing concern over Russia's moves to support Syrian President Bashar al-Assad and warned that military backing for his regime risks further hampering efforts at bringing peace.

Moscow has been on a diplomatic push to get the coalition of Western and regional powers fighting IS in Syria to join forces with Assad against the jihadists.