Three children are among the dead, Syrian opposition media reported, adding that more than 15 others were also wounded in the incident.
Tens of others are still trapped under the rubble while efforts are underway by firefighters to save their lives, according to social network activists.
No one or group has claimed responsibility for the explosion yet.
Another explosion in Idlib, northwestern Syria, killed five, leaving 43 others wounded last week.
During unrests in Idlib over the past few weeks, some people were killed and some others were kidnapped.
Apart from Fu'ah and Kefriya towns, all other parts of Idlib are controlled by terrorist groups.
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