An early morning explosion at a building thought to be used by an arms dealer killed at least 42 people, including 12 children, in Syria's rebel-held Idlib province on Sunday.
The Britain-based Syrian Observatory for Human Rights said the blast occurred in the town of Sarmada near the Turkish border.
The group, which monitors the conflict with the help of a network of contacts in Syria, said dozens of people are still missing and the toll was expected to rise.
The observatory said the building was occupied by people displaced by the war from the central Homs province.
It said an arms depot in the basement of the building had detonated, causing the entire multi-story structure to collapse.
Meanwhile, Syrian government forces have sent more reinforcements ahead of a potential offensive on Idlib province, the last major rebel stronghold in Syria.
The pro-government Al-Watan daily said Sunday that military reinforcements have reached the outskirts of Idlib province as a preliminary step to launch a wide-scale offensive.
Last week, government helicopters dropped leaflets over towns in Idlib's eastern countryside urging people to surrender.
At the same time, Jan Egeland, head of the United Nation's humanitarian task force for Syria appealed for talks to avert "a civilian bloodbath" in the province.
More than 350,000 people have been killed and millions displaced since Syria's civil war started in 2011 with the brutal repression of anti-government protests.