iStock/Thinkstock(LONDON) — The death toll from an explosion in an apartment building in rebel-held northwestern Syria has risen to 69, including 17 children, a monitoring group said on Monday.

The explosion happened on Sunday morning in the town of Sarmada, when a weapons depot in the basement of an apartment building detonated, according to the U.K.-based Syrian Observatory for Human Rights. Most of the residents in the building had been displaced from Syria’s Homs province, the organization said.

Fifty-two of those killed were civilians, including 17 children, and the rest were mostly rebel fighters, the monitoring group said.

The White Helmets, a Syrian search-and-rescue group, gave a slightly lower toll, saying 67 people were killed and another 37 injured in the explosion. The group said the explosion destroyed two buildings and that the search-and-rescue operation took 22 hours.

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