Commenting on the explosion in Jisr al Shughur town of Idlib province, chief of the Syrian Observatory for Human Rights Rami Abdel Rahman says "it is not known until now whether it was a car bomb, or the explosion of a car carrying explosives."
At least 17 people were killed on Wednesday in an explosion in a rebel-held city in northwestern Syria, a day after heavy Russian air strikes, rescue workers and residents said.
The cause of the blast in the town of Jisr al Shughur was not immediately clear, the Syrian Observatory for Human Rights said. The Britain-based war monitor had earlier announced the death toll as 15.
"The explosion hit next to the market," killing 13 civilians, including the daughter of a foreign fighter, Observatory chief Rami Abdel Rahman said.
"It is not known until now whether it was a car bomb, or the explosion of a car carrying explosives," he added.
Several buildings collapsed as a result of the blast which shook the city in Idlib province, near a road between the coastal city of Latakia and city of Aleppo.
The city has been a target of bombardment by the Russian air force and the regime forces in recent weeks. Most of its inhabitants have fled to the safety of areas close to the Turkish border, residents and local officials say.
Idlib region is under the administrative control of Hayat Tahrir al Sham, Syria's former Al Qaeda affiliate.
Daesh has sleeper cells in the wider Idlib region.
Idlib has since September been protected from a massive regime offensive by a fragile ceasefire deal signed by Russia and Turkey.
But the region of some three million people has come under increasing bombardment since HTS took control of it in January.
On Tuesday, regime shelling killed seven civilians, including four children, in the town of Khan Shaykhun.
Increased regime shelling on Khan Shaykhun has sparked one of the largest waves of displacement since the September deal.
Earlier this month Turkish President Erdogan speaking at a news conference with his Russian counterpart Vladimir Putin in Kremlin said that Ankara continues to take measures on Idlib since it is a "sensitive" issue for Turkey, Erdogan said.
He stressed that Idlib must be completely cleared of terrorism for people to return safely to their homes.
Syria's war has killed more than 370,000 people and displaced millions since the conflict began with the repression of anti-regime protests in 2011.