An explosion at a weapons depot in a rebel-held town in northwest Syria killed at least 39 civilians including a dozen children Sunday, 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", Mr 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 news agency that women and children were among the dead. But rescue workers had pulled out "five people who were still alive", the source said.