Up to 50 people have been killed in missile attacks on schools and hospitals in northern Syria, according to the United Nations (UN).
"Such attacks are a blatant violation of international laws," the UN said.
Among the sites hit was a Medecins Sans Frontieres (MSF) 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".
MSF blamed Syria's government or Russia for the raid in Maarat al-Numan. There has been no independent confirmation.
A further eight people are missing from the strikes, the medical charity said.
Separate strikes on a hospital in the same town, as well as on two more hospitals and a school in Azaz, killed at least 12 people, reports said.
Attacks on medical facilities are forbidden in conflict zones under international humanitarian law.
The raids in northern Syria come days after Russia and other world powers agreed to a limited cessation of hostilities, to begin later this week.
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.
MSF, also known as Doctors Without Borders, said its hospital in Maarat al-Numan was hit by four missiles within minutes of each other.
This "leads us to believe that... it wasn't an accidental attack, that it was deliberate," said Sam Taylor, the spokesman for MSF operations in Syria.
Mego Terzian, president of MSF France, told Reuters "either the [Syrian] government or Russia" was "clearly" responsible.
Monday's attack in Idlib province leaves tens of thousands without medical care, MSF warned.
Hospital and school struck in Azaz
In Azaz, near the Turkish border, at least 12 people were killed in an attack on a children's hospital and school, according to activists and witnesses.
One medic, Juma Rahal, told the Reuters news agency: "We have been moving scores of screaming children from the hospital".
There has been no independent confirmation over who was behind either attack in Azaz or Maarat al-Numan.
Unicef said four medical facilities across Syria had been struck: two in Idlib province, two in Azaz. Two schools in Azaz were also hit, the organisation said, reportedly killing six children.
Turkish Prime Minister Ahmet Davutoglu said a Russian ballistic missile had hit buildings in Azaz, with children among the dead.
Russia has been carrying out air strikes in Syria since September in support of President Assad and against what it terms "terrorists".
For a third day, Turkey has shelled Kurdish forces in northern Syria trying to take Azaz.
The Kurdish YPG militia has been making advances at the expense of other rebels stretched by a government offensive.
Ahmet Davutoglu promised the "harshest reaction" if the YPG tried to capture Azaz.
Turkey views the YPG militia in Syria as allied to the outlawed PKK, which has carried out a decades-long campaign for Kurdish autonomy within Turkey.
Syria said the Turkish shelling was a violation of its sovereignty and has called on the UN Security Council to act.
Last Thursday, 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.