The explosion took place in an ammunition depot belonging to the Al Qaeda-linked Nusra Front, according to the Syrian Observatory for Human Rights.
The depot was located on the bottom floor of a residential building, which was completely destroyed during the explosion, the London-based watchdog said.
Many people were buried under the rubble, most of whom were displaced people from the central province of Homs, it added.
The death toll could further rise as many are critically wounded and still trapped under the rubble.
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