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Buildings, one with basement arms depot, demolished in residential area of town in Idlib province

Workers from Syria Civil Defence, also known as the White Helmets, survey the scene of an explosion that brought down two buildings in the town of Sarmada, near the Turkish border, in northern Syria. (Syria Civil Defence via AP)

An explosion in northern Syria killed at least 36 people Sunday and wounded many others, but the cause of the blast wasn't immediately known, opposition activists said.

The opposition-run Syria Civil Defence, first responders also known as the White Helmets, said the blast occurred in the town of Sarmada near the Turkish border. The explosion collapsed two five-storey buildings, burying many of the victims, it said.

The Britain-based Syrian Observatory for Human Rights put the death toll at 39, including 21 women and children.

Destroyed buildings are seen following an explosion at an arms depot in a residential area of Sarmada. (Omar Haj Kadour/AFP/Getty Images)

An opposition media collective known as the Smart news agency, said the dead included civilians as well as members of the al-Qaeda-linked Levant Liberation Committee.

The Observatory said an arms depot in the basement of a building had detonated. It said the depot was run by an arms dealer close to the Levant Liberation Committee.

Meanwhile, Syrian government forces fighting rebels in Idlib province have sent more reinforcements ahead of a potential offensive on the last major rebel stronghold in Syria.