At least 17 people were killed on Wednesday in an explosion in the center of Jisr al-Shughour, an opposition-held city in northwestern Syria. Several residential buildings collapsed as a result of the blast in Idlib province, near a road between the coastal city of Latakia and city of Aleppo. “The casualties are expected to rise. The cause is not known,” said Ahmad Yaziji, head of civil defense in the city, adding at least 27 people, mostly civilians, were injured. Bodies were still being pulled out of the rubble, another rescuer said. Thirteen 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," killing 17 people including the daughter of a Turkestani fighter, Observatory chief Rami Abdel Rahman said. He 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. Abu Ammar, a father of two, told AFP he felt the "huge" blast from his home about 50 metres (yards) away. "We ran to the place of the explosion and saw the rescue teams trying to pull out the wounded," he said. The province and areas around it in northern Syria, the last remaining opposition bastion, have seen an escalation in attacks by Russian warplanes and the Syrian regime even though they are protected by a “de-escalation zone” agreement brokered last year between Russia, Iran and Turkey. The bombardment has sent people fleeing from opposition-held towns in the buffer zone that straddles parts of Idlib to northern Hama and parts of Latakia province. Turkey, which has supported the opposition and has troops to monitor the truce, has been negotiating with Moscow to halt the Russian strikes with little success. Russian jets hovered in the sky with missile strikes reported in several villages around Jisr al-Shughour, residents said. Overnight, Russian jets struck western parts of Idlib city and residents said long-range missiles hit several villages surrounding Jisr al-Shugour from regime positions in Latakia province. Jisr al-Shughour has been a target of heavy bombardment by the Russian air force and the Syrian regime in recent weeks. Most of its inhabitants have fled to the safety of areas close to the Turkish border, residents and local officials say.