Up to 50 people have been killed in missile attacks on schools and hospitals in northern Syria, according to the United Nations.
"Such attacks are a blatant violation of international laws," the UN said.
Among the sites hit was a Medecins Sans Frontieres hospital, where seven people were reportedly killed. France said such acts constituted war crimes.
Activists have accused Russia of carrying out the strikes but there has been no independent confirmation.
Russia has been backing the Syrian government in its offensive against rebels but says it only targets what it calls "terrorists".
'Deliberate' attack on MSF hospital
Two medical facilities in Maarat al-Numan, which is in Idlib province, are reported to have been hit.
MSF said one of its facilities had been struck by four missiles in the space of minutes, leading them to believe it "wasn't an accidental attack, that it was deliberate".
It said seven people died with another eight still missing.
Mego Terzian, president of MSF France, told Reuters "either the [Syrian] government or Russia" was "clearly" responsible.
But the Syrian ambassador to Moscow Riad Haddad, said the US was to blame, a claim the Pentagon dismissed as "patently false".
"We have no reason to strike in Idlib, as Isil [Islamic State] is not active there," spokesman Capt Jeff Davis said.
What does the law say about bombing hospitals?
- International humanitarian law bans any attack on patients and medical personnel or indeed any attack on medical facilities, which are zones that must be respected under the rules of war
- Even if combatants take refuge in them, they should not be attacked
- Under rules established by the International Criminal Court, any such incident would probably result in too high a number of civilian casualties - what is called the rule of proportionality
A second hospital in Maarat al-Numan was also hit, killing three people, said opposition group the Local Co-ordination Committees.
The strikes follow a pattern of systematic attacks on healthcare facilities in Syria, says the BBC's Mark Lowen in neighbouring Turkey.
Hospitals and schools struck in Azaz
In Azaz, near the Turkish border, at least 12 people were killed in an attack on two hospitals and two schools, reports said.
One of those hit was a children's hospital. A worker for Syria Charity, which runs the facility, blamed Russia.
"The Russians have been targeting this area because it's what we call a liberated area, by moderate opposition - that's why we are 99% sure this was Russian airstrikes," said Anfal Sevik.
Unicef said six children were killed in the strikes on schools.
"Let us remember that these victims are children," a statement said. "Children."
Azaz has been the focus of intense fighting, with Turkey on Monday threatening Kurdish rebels with the "harshest reaction" if they tried to take the town.
Despite the bombardment Kurdish-led forces have captured the town of Tal Rifaat from Islamist rebels, the monitoring group the Syrian Observatory for Human Rights said.
Doubts over 'cessation'
A statement from UN spokesman spokesman Farhan Haq said the attacks "cast a shadow" on commitments made by international powers last week.
At a conference, world leaders pledged to work towards a cessation of hostilities in Syria within a week.
But Russia argues that the "cessation" does not apply to its air strikes, which have tilted the balance of the war in favour of the Syrian government.
Syrian President Bashar al-Assad said any ceasefire did not mean "each party will stop using weapons".
In televised comments he questioned whether conditions for the halt in fighting could be met in a week, Reuters reported.
'War crime' says France
France said it condemned the bombing of the MSF clinic in the strongest terms, with Foreign Minister Jean-Marc Ayrault saying such acts "constitute war crimes".
The US has also condemned the strikes, 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".
EU foreign policy chief said the attack on the MSF facility was "completely unacceptable" but did not say who was responsible.
The UN envoy to Syria, Staffan de Mistura, is in the capital Damascus as part of his effort to restart peace talks.
Almost five years of civil war in Syria have led to the deaths of more than 250,000 people. More than 11 million people have been displaced.