An explosion at a weapons depot in a rebel-held town in northwest Syria killed at least 39 civilians including a dozen children, a monitor said.
The explosion happened in the town of Sarmada, which is near the Turkish border and north of the provincial capital Idlib city.
Rami Abdel Rahman, head of the Syrian Observatory for Human Rights monitoring group, said a previous death toll of 12 increased after more bodies were retrieved from the rubble.
But the cause of the blast was "not yet clear", Abdel Rahman added.
He said most of those killed were family members of fighters from Hayat Tahrir al-Sham (HTS), an alliance led by jihadists from Syria's former Al-Qaeda affiliate, who had been displaced to the area from the central province of Homs.
A civil defence source told AFP that women and children were among the dead. But rescue workers had pulled out "five people who were still alive", the source said.
Most of Idlib is controlled by rebels and HTS, but Isis (Islamic State) also has sleeper cells in the area.
In recent days, regime forces have ramped up their deadly bombardment of southern Idlib and sent reinforcements to nearby areas they control.
At the weekend, 12 civilians, three of them children, were killed in regime bombardment of the towns of Khan Sheikhun and Al-Tah.
Bashar al-Assad, the Syrian President, has warned that government forces intend to retake Idlib, after his Russia-backed regime regained control of swathes of rebel-held territory in other parts of Syria.
Last week, government helicopters dropped leaflets over towns in Idlib's eastern countryside urging people to surrender. The United Nations appealed the same day for talks to avert "a civilian bloodbath" in the province.
Jan Egeland, head of the UN's humanitarian taskforce for Syria, said: "The war cannot be allowed to go to Idlib."
Around 2.5 million people live in the province, half of them displaced by fighting in other regions of the country.