Syrian opposition activists said an explosion in the country's north has killed at least 18 people and wounded many others.
The Observatory said it was not clear if the Syrian regime or its Russian backers were responsible for the bombardment of Aleppo, but blamed the allies for pounding Idlib.
Several people remained trapped in the collapsed building for hours, with local reports saying that 10 people had been pulled from the rubble alive, while dozens of people remain missing.
Most of the residents of the building were Syrians who were displaced from the Homs province.
One of the buildings was scorched black after a fire followed the explosion.
Most of the rest is held by rebels, while the regime also holds a slither of the province's southeast.
Syrian civil defense members conduct search and rescue operations at wreckage of a building after a blast in Idlib, Syria.
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In recent months, a series of explosions and assassinations - mainly targeting opposition officials and fighters - have rocked the province.
Idlib is the last opposition bulwark in Syria, controlled by a patchwork of mainstream rebel groups and Islamist factions including...
While some attacks have been claimed by ISIS, most are the result of infighting since past year between other groups. They are considered a terrorist organization by Russian Federation and the Syrian regime.
On Thursday, government helicopters dropped leaflets over towns in Idlib's eastern countryside urging people to surrender.
At the same time, Jan Egeland, head of the United Nation's humanitarian task force for Syria appealed for talks to avert "a civilian bloodbath" in the province.
The war in Syria has killed more than 500,000 people since it began in 2011 with a brutal regime crackdown on protesters.