BEIRUT: Syrian government troops and allied militia backed by Russian airstrikes launched an offensive against rebels in the Ghab Plain in western Syria Thursday, the Britain-based Syrian Observatory for Human Rights and a rebel fighting there said.

Insurgents advanced into the area in July, building on gains that posed a threat to the coastal region vital to President Bashar Assad's control of western Syria and catalyzing Russia's intervention on his side.

Rami Abdulrahman, director of the Observatory, told Reuters that ground forces were targeting insurgent-held areas with heavy barrages of surface-to-surface missiles, as Russian jets bombed from above.

Abdulrahman, whose organisation uses sources on the ground to track the conflict, also said that an offensive launched by the Syrian army and its foreign allies on Wednesday in nearby areas of Hama province had failed to make significant gains.

"Thirteen regime forces were killed. There are only seven confirmed dead among the (rebel) fighters, but the number is certainly more than seven," he said.

The Ghab Plain abuts a mountain range that forms the heartland of Assad's Alawite sect. An alliance of rebel groups including al Qaeda's Nusra Front thrust into the area in late July, forcing government forces to retreat.

Syrian troops and militia backed by Russian warplanes mounted what appeared to be their first major coordinated assault on Syrian insurgents on Wednesday in nearby areas of Hama province.

Russian airstrikes started last week and have mostly focused in areas of western Syria where Assad has sought to shore up his control after losing swathes of the rest of the country to insurgents including ISIS.

Russia says it is fighting ISIS in Syria. But while ISIS has been the target of some of its airstrikes, it has no foothold in the areas of western Syria targeted in the attacks on Wednesday and Thursday.

Abu al-Baraa al-Hamawi of the Ajnad al-Sham insurgent group told Reuters that Russian jets had been bombing since dawn. It was not the first time the Russians had bombed the area, but this was their most ferocious attack, he said, speaking via an Internet messaging service.

"There is an attempt by the regime to advance but the situation is under our control," he said.