By Burak Karacaoglu & Esref Musa

IDLIB, Syria

An explosion in Syria’s northwestern city of Idlib killed at least 15 civilians and injured 36 others, many of them children, a local civil defense official reported Monday.

Moustafa Haj Youssef, the director of the White Helmets civil defense agency in Idlib, told Anadolu Agency that the cause of the blast, which also damaged nearby houses and vehicles, was yet to be known.

Search and rescue efforts were underway at the site of the blast that leveled a seven-story building, Youssef added.

Controlled by anti-regime armed groups, Idlib has been under intense airstrikes for the last two months, which caused hundreds of deaths and injuries, according to civil defense sources.

Located in northern Syria near the Turkish border, Idlib was declared last year a "de-escalation zone" where acts of aggression are forbidden.

Syria has been locked in a devastating civil war since March 2011, when the Bashar al-Assad regime cracked down on pro-democracy protests with unexpected ferocity.

UN officials say hundreds of thousands of people have been killed in the conflict.