A Syrian search-and-rescue group says the death toll from an explosion that destroyed two apartment buildings in a rebel-held town in the country's northwest the previous day has risen to 67.

The Syria Civil Defence, also known as the White Helmets, says rescuers were still searching through the rubble in Sarmada on Monday, looking for survivors from the blast. It says 35 wounded people have already been found.

The Britain-based Syrian Observatory for Human Rights gave a slightly higher death toll, saying 69 had died, including 17 children. The Observatory says 52 of the victims were civilians; the rest were militants or couldn't be identified.

The Observatory said an arms depot in the basement of a building had detonated, bringing down two five-story apartment buildings. The report could not be independently confirmed.

On Sunday, an AFP correspondent at the site in Sarmada in Idlib province near the Turkish border said the explosion of unknown origin caused the collapse of two buildings.

The explosion also killed 17 members of Hayat Tahrir al-Sham (HTS), a militant-led alliance, according to the Britain-based monitor, which relies on a network of sources inside Syria.

“Rescue operations are still ongoing,” Observatory head Rami Abdel Rahman told AFP, more than 24 hours after the blast at the depot inside a residential building.

Most of the civilians killed were family members of HTS fighters displaced to the area from the central province of Homs, he said.

HTS controls more than half of Idlib province and is led by militants from Syria's former Al-Qaeda affiliate.

Most of the rest is held by rebels, while the regime also holds a slither of the province's southeast.

The militant Islamic State group also has sleeper cells in the area.

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 IS, most are the result of infighting since last year between other groups.

Government forces have in the past week ramped up their deadly bombardment of southern Idlib and sent reinforcements to nearby areas they control.

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.

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