Syrian planes are pounding the Islamic State-held town of Palmyra, killing at least 15 people in a series of air strikes, according to activists.

They say the attacks are some of the heaviest since the extremist group seized the ancient town in central Syria on May 10. A local activist reported around 30 air strikes.

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

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.

Moscow said it would consider sending troops to fight in Syria if Damascus asked for them, a spokesman for president Vladimir Putin said.

Dmitry Peskov spoke in response to comments by Syrian foreign minister Walid al-Moallem, who denied reports that Russian combat troops were fighting with Syrian forces, but said Syria would ask for Russia's help if needed.

If such a request is made, it will be "discussed and considered", Mr Peskov said.