The Syrian Observatory for Human Rights war monitor said 13 of those slain were killed in regime air raids on the village of Jabala in Idlib province.

The Idlib region, dominated by an alliance led by Syria's former Al-Qaeda affiliate, is supposed to be protected by a months-old buffer zone deal.

That region includes most of Idlib province as well as adjacent parts of Latakia, Hama and Aleppo governorates.

But it has come under deadly regime bombardment in recent weeks, sparking fears for its roughly three million residents.

The spike in violence has killed more than 360 civilians since the end of April, says the Britain-based Observatory.

The violence has also forced more than 270,000 people to flee their homes and hit 24 health facilities, the United Nations says.

Analysts predict the regime will continue to chip away at the region, but say it is unlikely to unleash a major assault.

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

The Observatory, which relies on sources inside Syria for its information, says it determines who carries out air strikes according to flight patterns, as well as aircraft and ammunitions used.