An explosion killed at least 13 civilians in Syria's northwestern rebel-held city of Idlib and wrecked a multi-storey building on Monday, a monitor said.
The blast injured 80 others, the Britain-based Syrian Observatory for Human Rights said.
The group, which relies on sources inside Syria for its information, said the death toll was likely to rise as several of the wounded were in serious condition and many others were trapped under the rubble.
The cause of the explosion was not clear.
An AFP correspondent saw a bulldozer in the dark near the collapsed building, as rescue workers searched for victims using lamps.
Idlib city is controlled by Hayat Tahrir Al Sham, a group led by Syria's former Al Qaeda affiliate.
Held by an array of extremists and rebels, the surrounding province of the same name is the last in Syria largely beyond government control.
More than 350,000 people have been killed in Syria's seven-year civil war since it started in 2011 with the repression of anti-government protests.
Trump talks of imminent 'military options' in Syria
Editorial: Assad is emboldened to kill by his backers
Russia and Syria blame Israel for deadly attack on Syria airbase