An explosion at a weapons depot in a rebel-held town in northwest Syria killed at least 39 civilians including a dozen children on Sunday, a monitor said.
An AFP correspondent at the site in Sarmada in Idlib province near the Turkish border said the explosion of unknown origin caused two buildings to collapse.
Rescue workers used a bulldozer to remove rubble and extract trapped people, the correspondent said.
Rami Abdel Rahman, head of the Syrian Observatory for Human Rights monitor, said a previous toll of 12 civilians killed increased after more bodies were retrieved from the rubble.
"The explosion occurred in a weapons depot in a residential building in Sarmada," said the head of the Britain-based monitor, which relies on a network of sources inside Syria.
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, an alliance led by jihadists from Syria's former al-Qaida affiliate, who had been displaced to the area from the central province of Homs.