Beirut (AFP) - Russian air strikes on Syria's last major rebel bastion Saturday were the "most violent" in a month since Damascus and its ally Moscow started threatening it with an imminent attack around a month ago, a monitor said.

Nearly 60 Russian air raids hit Idlib province near the Turkish border in less than three hours, the Syrian Observatory for Human Rights monitoring group said.

The Russian strikes and regime barrel bombing on south and southeastern areas of the province killed at least four civilians including two children, the Britain-based monitor said.

The raids targeted jihadist and rebel positions, some of which were empty and others in use, Observatory head Rami Abdel Rahman said.

It was the "most violent" bombardment since August 10, when fierce bombardment killed at least 53 civilians including 28 children in Idlib and the neighbouring province of Aleppo, he said.

Idlib and nearby areas are largely controlled by Hayat Tahrir al-Sham, an alliance led by Al-Qaeda's former Syrian affiliate, as well as rival rebels.

On Friday, Russian air strikes killed four hardline rebels and a shepherd in Idlib province, the Observatory said.

The spike in violence came after Russia, fellow regime ally Iran and rebel backer Turkey on Friday failed to immediately agree on a solution to avert an imminent government offensive.

President Bashar al-Assad's regime has upped its rhetoric on retaking control of Idlib and surrounding areas over the past month.

The threats come after government forces seized back areas around the capital Damascus and in the south from rebels and jihadists earlier this year.

More than 350,000 people have been killed and millions displaced since Syria's civil war started in 2011 with the brutal repression of anti-Assad protests.