As many as 50 civilians were killed in airstrikes on seven schools and hospitals Monday in the Syrian provinces of Aleppo and Idlib, the United Nations said.
"Such attacks are a blatant violation of international laws," Farhan Haq, a spokesman for U.N. Secretary-General Ban Ki-moon, told reporters at a briefing Monday afternoon in New York.
Haq said children were among the dead in the missile strikes on at least five medical facilities and two schools in Aleppo and in Idlib.
The U.S. State Department blamed the regime of Syrian President Bashar Assad and his Russian backers.
"That the Assad regime and its supporters would continue these attacks, without cause and without sufficient regard for international obligations to safeguard innocent lives, flies in the face of the unanimous calls" by the International Syria Support Group to protect civilians, State Department spokesman John Kirby said in a statement.
That was a reference to an agreement reached last week in Munich, Germany, for nationwide "cessation of hostilities" in Syria and diplomatic talks aimed at ending the years-long conflict, which has intensified since the United Nations tried to revive peace talks. The talks were suspended this month.
The attacks "cast doubt on Russia's willingness and/or ability to help bring to a stop the continued brutality of the Assad regime against its own people," Kirby said.
Moscow has said it is targeting "terrorist groups" and has dismissed suggestion it has killed civilians since it began its air campaign in support of Assad in September.
Russian Prime Minister Dmitri Medvedev told Time magazine in an interview Saturday that Moscow has no plans to stop its bombing campaign against rebel positions in Syria until its allies in Damascus can achieve peace on favorable terms.
Among the targets in Monday's attacks was a hospital in the Idlib town of Maaret al-Numan supported by Médecins Sans Frontières, the international aid group known in English as Doctors Without Borders.
At least seven people were killed and eight others were missing and presumed dead in what the group's head of mission, Massimiliano Rebaudengo, called "a deliberate attack on a health structure."
MSF medical facilities are protected under international law.
The group said Monday that in addition to the Idlib airstrike there were reports that two hospitals unaffiliated with the group had been attacked in neighboring Aleppo province.
The Associated Press reported that at least five people were killed when a missile struck a hospital for women and children in Azaz.
Alaaeddin Alzaeem, who went to help with rescue efforts at the hospital, said, "It's broken in the hospital — everything."
Alzaeem also told NBC News by Skype that Russian warplanes were to blame.
"The streets, all, it's empty because Russians target Azaz," he said. "Russia and the regime killing us every day, and nobody do anything."