Six children were among the dead in the northwest province of Idlib, where a fresh ceasefire brokered by Russia and Turkey is expected to go into effect after midnight, according to the Syrian Observatory for Human Rights.

Air strikes on the city of Idlib killed seven civilians, while separate raids on two towns near the provincial capital killed 11 others, the Britain-based war monitor said.

In Idlib city, the bombardment hit near a cultural centre, according to the Observatory and an AFP correspondent in the area.

Scores of students, many of them crying, ran from the site of the blast in panic, the AFP correspondent said.

The bombardment surprised residents in a city that has been relatively free from the near-daily attacks that have hit the province's flash-point south, the correspondent added.

Less than 10 kilometres (six miles) away, regime air strikes hit a market in the town of Binnish, killing seven, according to the Observatory.

The market was mostly reduced to rubble, as thick white smoke from the strikes created a fog, according to an AFP correspondent there.

He saw men carrying what the apparently lifeless bodies of two children from the scene, as women and children wailed.

One woman pressed two children to her chest as she screamed for help, he said.

Volunteer rescue workers, meanwhile, carried bodies to an ambulance, and sifted through rubble searching for trapped civilians, he added.

South of Idlib city, raids hit the area of Al-Nerab, killing four.

"All of these areas where far away from bombardment during the latest (regime) offensive but today, regime warplanes," have returned, said Observatory head Rami Abdul Rahman.

A southern strip of the jihadist-dominated Idlib region has come under mounting bombardment in recent weeks, displacing more than 300,000 people in December alone, according to the United Nations.

The Damascus government has repeatedly vowed to retake Idlib, which is run by Hayat Tahrir al-Sham, a group dominated by Syria's former Al-Qaeda affiliate.

A ceasefire announced in late August was supposed to stop Russia-backed regime bombardment of the region after strikes killed some 1,000 civilians in four months.

But the Observatory says sporadic bombardment and clashes continued, before intensifying in the past month.

Syria's war has killed more than 380,000 people including over 115,000 civilians since it started in 2011 with the brutal repression of anti-government protests.