Two of the victims were killed in a southern area of Idlib province while the other 10 died in a western region of neighbouring Aleppo province, the Syrian Observatory for Human Rights said.

The raids on Aleppo province killed eight members of the same family who were sheltering in a house, Observatory chief Rami Abdel Rahman said.

Six children were among those killed in the raid on the village of Kfar Taal, where three girls had already died a day earlier in strikes, according to the Britain-based monitor.

Most of Idlib province and parts of Aleppo province are still controlled by factions opposed to the regime of President Bashar al-Assad, including a group that includes onetime members of Al-Qaeda's former Syria franchise.

"Over the past three days, the bombardment on Idlib and its surroundings, including in western Aleppo, has been exclusively Russian," Abdel Rahman said.

The surge in violence comes despite a ceasefire announced by Russia earlier this month that never really took hold.

- Imminent offensive? -

"They want to push rebels and jihadists away from the city of Aleppo and from the motorway linking Aleppo to Damascus," Abdel Rahman said.

He predicted that the air strikes could be a prelude to a land offensive in western Aleppo, as the regime and its allies continue their drive to shrink the country's last opposition-held pocket.

"The regime has massed reinforcements on the outskirts of the city of Aleppo," Abdel Rahman said.

Russia has thousands of forces deployed across Syria in support of the army, while an unknown contingent of Russian private security personnel also operates on the ground.

Moscow's military intervention in 2015, four years into the Syrian conflict, helped keep Assad in power and marked a long, bloody reconquest of the territory lost to rebels in the early stages of the war.

The violence in northern Syria is escalating an already dire humanitarian situation, with aid groups warning of displacement on an unprecedented scale.

Idlib province alone is home to at least three million people, many of whom are in need of urgent humanitarian assistance.

According to the UN humanitarian coordination agency OCHA, almost 350,000 people have fled their homes since December 1, mainly heading northwards from southern Idlib, which has borne the brunt of the air strikes.

The International Rescue Committee has warned that an additional 650,000 people, the majority of them children and women, could be forced from their homes if the violence continues.