At least 15 civilians were killed on Wednesday in an explosion in the center of Jisr al Shughour, a city in northwestern Syria, a day after heavy Russian airstrikes, rescue workers and residents said.

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.

Fifteen civilians were among those killed in the blast, the cause of which was not immediately clear, according to the Syrian Observatory for Human Rights, a Britain-based monitoring group.

"The explosion hit next to the market," Observatory chief Rami Abdel Rahman said.

Abdel Rahman earlier said the explosion could have been the result of a car bomb or a vehicle carrying explosives that detonated.

But bystanders and the head of the local civil defense unit, Abdelwahab al-Abdu, said they did not know what caused it.

Abdu said the civilians had died in "an explosion of unknown origin."

The Idlib region is under the administrative control of Hayat Tahrir al-Sham (HTS), Syria's former al-Qaida affiliate.

Idlib has since September been protected from a massive regime offensive by a cease-fire deal signed by Turkey and Russia.

But the region of some 3 million people has come under increasing bombardment since HTS took full control of it in January.

On Tuesday, regime shelling killed seven civilians, including four children, in the town of Khan Sheikhun.

Increased regime shelling on Khan Sheikhun has sparked one of the largest waves of displacement since the September deal.

Syria's war has killed more than 370,000 people and displaced millions since the conflict began with the repression of anti-government protests in 2011.