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 United Kingdom -based Syrian Observatory for Human Rights.
"The toll rose after the discovery of new victims under the rubble and the death of some of those who had been wounded", Observatory director Rami Abdel Rahman.
Three members of the jihadist group Hayat Tahrir al-Sham were also killed, he said.
White helmet rescue workers pulled survivors from the rubble following the blast.
Civil defense workers fight a fire at the scene of the explosion in Sarmada on Sunday.
They said the cause of the blast, which brought down the whole multi-story building block, was unknown.
Most of the rest of Idlib is held by opposition forces, while the regime also holds a slither of the province's southeast.
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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.
Numerous victims were women and children buried in the rubble after the explosion collasped two five-storey buildings in the village of Sarmada near the Turkish border.
While some attacks have been claimed by Daesh, most are the result of infighting since a year ago between other groups.
In recent days, regime forces have also ramped up their deadly bombardment of southern Idlib and sent reinforcements to nearby areas they control. They are considered a terrorist organization by Russian Federation and the Syrian regime.
A full-fledged assault would be devastating for the estimated 2.5 million people living in Idlib, many of them rebels and civilians bussed out of other areas that came back under regime control.
At least 53 civilians including 28 children, have been killed in fierce bombardment Friday that targeted rebel-held areas of northern Syria, a war monitor said updating an earlier toll.