DAMASCUS, Aug. 12 (CGTN) -- 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.

It is reported that the explosion of unknown origin caused two buildings to collapse.

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 the fighters from Hayat Tahrir al-Sham, 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.

Behind mounds of rubble, the facade of a building was scorched black, due to a fire after the blast.

In recent months, a series of explosions and assassinations – mainly targeting rebel officials and fighters – have rocked the province.

While some attacks have been claimed by ISIL, most are the result of infighting since last year between other groups.

Around 2.5 million people live in the province, half of them displaced by fighting in other parts of the country.

More than 350,000 people have been killed and millions displaced since Syria's civil war started in 2011 with the brutal repression of anti-government protests.