Russian-led air strikes killed at least 18 people – including an entire family – in north-west Syria, where a major government offensive to clear out rebels has sent tens of thousands of people fleeing toward the border with Turkey, residents and rescuers have said.

The family of eight including six children were killed in the rural village of Kfar Taal, west of government-controlled Aleppo, while another four civilians were killed in Maardabseh in the south-east of Idlib province.

“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.

At least eight other civilians were killed in other strikes by Russian and Syrian government warplanes on rural opposition areas that have been hit hard since the Russian-led military campaign, supplemented by Iranian militias, began in December.

It has left dozens of towns in ruins and knocked down hospitals and schools, UN aid agencies said.

UN officials said last week a humanitarian crisis in the Idlib region of far north-western Syria, the last significant rebel redoubt in Syria after almost nine years of civil war, had worsened with at least 350,000 civilians now on the run.

Another half a million people fled earlier bouts of fighting to the safety of camps near the border of Turkey, which backs some rebel factions in the north-west.

The latest offensive has brought Bashar al-Assad’s military campaign closer to heavily populated central areas of Idlib, where nearly 3 million people are trapped, according to aid charities and UN agencies.

Moscow and Damascus deny accusations of indiscriminate bombing of civilians, and have said they are fighting jihadist militants who have stepped up attacks on civilians in Aleppo city in northern Syria.

Syrian state television said two women and a child were killed in a rocket attack by “terrorists” – its standard term for anti-Assad rebels – on a crowded neighbourhood of Aleppo.