The number of people killed when an explosion ripped through a building storing arms in the rebel-held province of Idlib in northwestern Syria has climbed to 69 including 17 children, the Syrian Observatory for Human Rights said Monday. The explosion, which happened in the town of Sarmada on Sunday, took the lives of 52 civilians, it said. The blast also killed 17 members of Hayat Tahrir al-Sham, according to the Britain-based monitor. "Rescue operations are still ongoing," Observatory head Rami Abdel Rahman told AFP, more than 24 hours after the explosion. Most of the civilians killed were family members of HTS fighters displaced to the area from the central province of Homs, he said. HTS controls more than half of Idlib province. Most of the rest is held by rebels, while the regime also holds a slither of the provinces southeast. The province forms part of the last major rebel stronghold in Syria. In recent months, a series of explosions and assassinations -- mainly targeting rebel officials and fighters -- have rocked the province. While some attacks have been claimed by ISIS, which has sleeper cells in the area, most are the result of infighting since last year between other groups. Regime forces have in the past week ramped up their deadly bombardment of southern Idlib and sent reinforcements to nearby areas they control. The head of the Syrian regime, Bashar al-Assad, has warned that his forces intend to retake control of Idlib, which now has a population of around 2.5 million people, half of whom have been displaced by fighting in other regions of the country.