Activists say Syrian warplanes are pounding the Islamic State-held town of Palmyra in central Syria, killing at least 15 people in a series of airstrikes.

They say Friday's airstrikes are some of the heaviest since the extremist group seized the ancient town May 10.

The Palmyra airstrikes come a day after the Syrian army carried out heavy air raids in the northern city of Raqqa, also held by the Islamic State group.

The Syrian government says it is the leading force fighting IS in Syria. Russia, a key ally of President Bashar Assad, is trying to convince the West of the need to work with Syria in the fight against IS.

A local activist who spoke on condition of anonymity for security reasons reported around 30 airstrikes on Palmyra Friday.