Russia-led air raids have killed at least 12 people in Syria where a large government offensive to clear out rebels has sent tens of thousands of people fleeing towards the border with Turkey, according to residents and rescuers.

On Tuesday, a family of eight, including six children, was killed in the rural village of Kfar Taal west of government-controlled Aleppo, while another four civilians were killed in Maardabseh southeast of Idlib province.

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"God take revenge on all tyrants. There is no one else left in my family," Abu Yasser, 71, a relative of the family wiped out in Kfar Taal, said in a voice recording sent to Reuters News Agency.

There were also reports of another air raid by Russian and Syrian government that killed at least eight other civilians in rural opposition areas that have been hit hard since the Russian-led military campaign, supplemented by Iranian militias, began in December.

Retaliatory rocket attacks, blamed on the rebel fighters, killed three more civilians - two women and a child - in the government-held city of Aleppo in northern Syria, state news agency SANA said.

Rising hostilities

The spike in violence in the neighbouring provinces of Aleppo and Idlib follow the so-far unsuccessful diplomatic attempts to reduce hostilities in the flashpoint region, with the latest truce, in theory, going into effect less than two weeks ago.

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

The Damascus government, which controls about 70 percent of the country after nearly nine years of war, has repeatedly promised to recapture the region.

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 attacks.

The International Rescue Committee has warned another 650,000 people could be forced from their homes if the violence continues.