Up to 50 people were killed in missile attacks on schools and at least four hospitals in the region on Monday.
Both France and Turkey said the strikes constituted war crimes. Turkey`s foreign ministry blamed Russia for the attacks.
The strikes came days after world powers - including Russia - agreed to work towards a selective truce in Syria, due to begin later this week.
The UN envoy to Syria, Staffan de Mistura, is in the capital, Damascus, to discuss the implementation of the "cessation of hostilities" - which could include the immediate delivery of humanitarian aid to besieged areas.
But Syrian President Bashar al-Assad has cast doubt over the plan, saying it did not mean all the parties would put down their weapons.
"So far they say they want a ceasefire within a week," he said in his first comments of Friday`s agreement. "Who is capable of gathering all these conditions and requirements within a week?"
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