BEIRUT, Aug 13 (Reuters) The number of people killed when an explosion ripped through a building thought to be storing weapons in rebel-held northwestern Syria has climbed to 69 including 17 children, the Syrian Observatory for Human Rights reported on Monday.

The explosion happened in a residential building in the town of Sarmada in Idlib province near the Turkish border on Sunday. Idlib forms part of the last major rebel stronghold in Syria.