The Syrian Observatory for Human Rights, a UK-based war monitor, said the building was believed to be storing weapons and ammunition.
An opposition media collective known as the Smart news agency, said the dead included civilians as well as members of the al-Qaida-linked Levant Liberation Committee.
On Sunday, rescuers in Sarmada used bulldozers to remove the rubble and pull out trapped people, an AFP correspondent in the town near the Turkish border reported.
Its head, Rami Abdel Rahman, said the cause of the blast was "not yet clear", and that most of those killed were family of fighters from HTS, a jihadists' alliance.
Idlib is the last opposition bulwark in Syria, controlled by a patchwork of mainstream rebel groups and Islamist factions including...
Most of the rest of Idlib is held by opposition forces, while the regime also holds a slither of the province's southeast.
Destroyed buildings from an explosion at an arms depot in a residential area in Syria's northern Idlib province city of Sarmada, Aug. 12, 2018.
The facade of a building behind the rubble was scorched black from the blast.
While some attacks have been claimed by IS, most are the result of infighting since a year ago between other groups.
Idlib is the last major rebel-held area, and is expected to be the next target for Syrian armed forces.
The death toll from an explosion at a weapons depot in northwestern Syria has risen to 69, mostly civilians including 17 children. They are considered a terrorist organization by Russian Federation and the Syrian regime.
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.
Rebels and civilians bussed out of the other three zones when they fell to the regime were taken to Idlib, increasing its population to around 2.5 million people.