An explosion at a weapons depot in a rebel-held town in north-west Syria killed at least 39 civilians, including a dozen children, according to reports.
Rescue workers used bulldozers to remove rubble and extract people trapped in the two buildings flattened by the explosion in Sarmada in Idlib province on Sunday.
A civil defence source told AFP that rescue workers had pulled out “five people who were still alive”.
But the death toll rose as more bodies were retrieved from the rubble, according to Rami Abdel Rahman, head of the UK-based Syrian Observatory for Human Rights monitoring group.
Three members of the jihadist group Hayat Tahrir al-Sham were also killed, he said.
“The explosion occurred in a weapons depot in a residential building in Sarmada,” said Abdel Rahman, whose group relies on a network of sources inside Syria.
The cause of the blast was “not yet clear”, he added.
Most of those killed were family members of fighters from HTS, 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.
A rescue worker carried the motionless body of a small child from the wreckage to an ambulance, an AFP correspondent at the scene said.
White Helmet rescue workers attempted to lift part of a floor of one of the buildings with a tall crane, as three young boys watched on in silence, perched on a rock.
Behind mounds of rubble, the facade of a building was scorched black, due to a fire after the blast.
Most of Idlib is controlled by rebels and HTS, but the Islamic State jihadist group also has sleeper cells in the area.
President Bashar al-Assad’s regime holds a small slither of south-eastern Idlib.
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.
Regime forces have since last week ramped up their deadly bombardment of southern Idlib and sent reinforcements to nearby areas they control.
On Friday, 12 civilians, three of them children, were killed in regime bombardment of the towns of Khan Sheikhun and al-Tah.
Assad has warned that government forces intend to retake Idlib, after his Russia-backed regime regained control of large parts of rebel-held territory in other parts of Syria.
On Thursday, government helicopters dropped leaflets over towns in the eastern part of Idlib 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.
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.