Part of the building in the town of Sarmeen had been turned into a shelter for the displaced, the Britain-based Syrian Observatory for Human Rights said.

Fifteen people were wounded, Observatory head Rami Abdel Rahman said.

An AFP correspondent in Sarmeen saw the remains of a missile several metres long in a nearby olive grove.

In the latest round of violence in Syria's nearly nine-year-old war, regime forces have upped their deadly bombardment of the northwestern opposition bastion of Idlib in recent weeks.

In December alone, the violence pushed some 284,000 from their homes in the jihadist-run region of some three million people, the United Nations says.

The mass movement of people has seen public buildings such as mosques, garages, wedding halls and schools turned into shelters, UN humanitarian agency OCHA says.

Regime ally Russia announced a ceasefire for Idlib in late August after months of deadly Russian and regime bombardment that killed around 1,000 civilians.

But sporadic clashes and bombardment persisted throughout the autumn before a spike in violence in the past month, the Observatory says.

Syria's civil war has killed more than 370,000 people since it started in 2011 with the brutal repression of anti-government protests.

In total 11,215 people including more than 1,000 children were killed during the war last year, although it was the least deadly year on record since the beginning of the conflict.