The residential building was thought to be storing weapons and ammunition, a war monitor said.
A tractor clears the rubble of destroyed buildings, August 13, 2018, following an explosion at an arms depot in a residential area in Syria's northern Idlib province city of Sarmada in which dozens of people were reportedly killed.
But the death toll rose as more bodies were retrieved from the rubble, according to Rami Abdel Rahman, head of the Syrian Observatory for Human Rights monitoring group.
White helmet rescue workers pulled survivors from the rubble following the blast. The group responds to aid and rescue other civilians when an explosion, fire or other attack occurs.
Most of the civilians killed were family members of HTS fighters displaced to the area from the central province of Homs, he said.
Idlib is the last opposition bulwark in Syria, controlled by a patchwork of mainstream rebel groups and Islamist factions including...
File: A picture taken on 29 May 2015 from the top of a hill shows a general view of the town of Ariha, in the northwestern province of Idlib, a day after the city was seized by a rebel coalition led by al-Qaedas Syria franchise.
While some attacks have been claimed by Daesh, most are the result of infighting since past year between various other groups.
Meanwhile, Syrian government forces are fighting rebels in Idlib province.
On Friday, 12 civilians, three of them children, were killed in regime bombardment of the towns of Khan Sheikhoun and al-Tah.
President Bashar al-Assad has warned that government forces intend to retake control of Idlib, after his Russia-backed regime regained chunks of territory from rebels and militants in other parts of Syria.
Jan Egeland, head of the UN's humanitarian taskforce for Syria, said the war "cannot be allowed to go to Idlib".
The war in Syria has killed more than 500,000 people since it began in 2011 with a brutal regime crackdown on protesters.