France and Turkey have said that air strikes on hospitals in northern Syria constitute war crimes.
Up to 50 people were killed in missile attacks on schools and hospitals in the region, the UN said.
Turkey's foreign ministry blamed Russia for the attacks. Moscow has yet to respond to the allegations.
Meanwhile, Syrian President Bashar al-Assad has cast doubt over plans to implement a "cessation of hostilities" in Syria.
Last week world powers agreed to work towards a selective truce in Syria, to begin later this week.
The UN envoy to Syria, Staffan de Mistura, is in the capital, Damascus, to discuss the implementation of the truce - which could include the immediate delivery of humanitarian aid to besieged areas.
Earlier, in his first comments on the announcement, President Assad said such a ceasefire 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 televised comments.
"Who is capable of gathering all these conditions and requirements within a week?"
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The UN said the raids in northern Syria "cast a shadow" on the prospects for a cessation.