Russian air strikes have killed 18 Islamic State group fighters in central Syria, including near the historic city of Palmyra, over the past 24 hours, a monitor said on Friday.

The Britain-based Syrian Observatory for Human Rights said European members of IS were among the dead in the strikes around the town of Sukhna and near Palmyra in the central province of Homs.

A key ally of the Syrian government, Russia began carrying out air strikes in the country in late September.

On Thursday, it said it had carried out more than 50 raids over two days targeting IS jihadists near Palmyra, which government forces recaptured from the extremist group in March.

"The Russian air force has increased the intensity of its strikes against targets of the armed units of international terrorist organisations near the city of Palmyra," the ministry said in a statement Thursday on its Facebook page.

Six Tupolev bombers flew out of an airbase in Russia on Thursday morning and conducted strikes east of Palmyra, near the cities of Arak and Sukhna, as well as in the Homs region, the statement said.

Both Arak, a small town that has strategic importance because of a nearby oilfield, and Sukhna are under IS control.

The Russian strikes come after an IS attack on Palmyra on Monday, which was repelled by government forces.

They also follow the deaths of two Russian pilots after a Syrian helicopter was downed by IS fighters in the area on Saturday.

Their deaths bring the number of Russian troops killed in the Syrian conflict to 12.