Close to 50 civilians have been killed and many more wounded in missile attacks on at least five medical facilities and two schools in northern Syria, UN Secretary-General Ban Ki-moon said.
UN deputy spokesman Farhan Haq said that victims of the attacks included children.
He quoted the secretary-general when he called the attacks "blatant violations of international laws" that "are further degrading an already devastated health care system and preventing access to education in Syria".
Mr Haq quoted Mr Ban as saying the attacks "cast a shadow on commitments" made by nations seeking to end the Syrian conflict at a conference in Munich on February 11, which included a cessation of hostilities within a week and an end to attacks on civilians.
In Idlib province, an airstrike destroyed a makeshift clinic supported by Doctors Without Borders.
The international charity, also known by its French acronym MSF, said the hospital in the town of Maaret al-Numan was hit four times in attacks that were minutes apart.
It said seven people were killed and eight others were "missing, presumed dead".
"The destruction of the hospital leaves the local population of around 40,000 people without access to medical services in an active zone of conflict," said MSF mission chief Massimiliano Rebaudengo.
The Britain-based Syrian Observatory for Human Rights said Russian warplanes targeted the hospital, destroying it and killing nine people.
The opposition group, which tracks both sides of the conflict through sources on the ground, said dozens were wounded in the attack.
"The entire building has collapsed on the ground," said opposition activist Yahya al-Sobeih, speaking by phone from Maaret al-Numan.
He said five people were killed near the MSF clinic and "all members of the medical team inside are believed to be dead".
The Observatory and other opposition activists said another hospital in Maaret al-Numan was also hit on Monday, most likely by a Syrian government airstrike.
In the neighbouring Aleppo province, a missile struck a children's hospital in the town of Azaz, killing five people, including three children and a pregnant woman, according to the Observatory.
A third air raid hit a school in a nearby village, killing seven and wounding others.
Activists posted an amateur video that showed civil defence workers pulling bodies from the rubble of the MSF-supported structure in Idlib.
Other images showed a huge crater next to a building that purportedly housed the child and maternal hospital in Azaz. Incubators could be seen in a ward littered with broken glass and toppled medical equipment.
Russia said its airstrikes are targeting militants and denied hitting hospitals or civilians. However, rights groups and activists have repeatedly accused Moscow of killing civilians.
Daragh McDowell, the head Russia analyst at Verisk Maplecroft, a British risk analysis firm, said Russian and Syrian tactics "strongly suggest a deliberate effort to further exacerbate the refugee crisis, as a means of destabilising Europe and pressuring the West to agree to a settlement in Syria on Moscow's and Damascus' terms".
Abdulrahman Al-Hassan, chief liaison officer at the Syrian Civil Defence, a group of first responders known as the "White Helmets", said the women's hospital in Azaz was hit by two surface-to-surface missiles. He said some 10 people were killed and many were wounded.
"We think it is Russia because the photos of the missiles have Russian language (and) because we haven't seen this kind (of missile) before the Russian intervention," he said.
Russia has been a key ally of Mr Assad throughout the five-year uprising and civil war, and began launching airstrikes on September 30.
The private Turkish Dogan news agency reported that more than 30 of those wounded in Russian airstrikes in Azaz, primarily children, were transferred to a hospital in southern Turkey. It showed footage of ambulances arriving and medics unloading children on stretchers.
The US State Department condemned the airstrikes, saying they 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".
In Brussels, European Union officials had earlier called on Turkey to halt its military action in Syria after Turkish forces shelled positions held by a US-backed Kurdish militia over the weekend.
The EU's foreign policy chief, Federica Mogherini, said that "only a few days ago, all of us including Turkey, sitting around the table, decided steps to de-escalate and have a cessation of hostilities".
Dutch foreign minister Bert Koenders, whose country holds the EU's rotating presidency, said "we have the plan for a cessation of hostilities and I think everybody has to abide by that".