Up to 50 people - including many children - were killed and dozens of others wounded in air raids on two schools and five hospitals in separate locations in northern Syria, UN Secretary-General Ban Ki-moon has said.
Farhan Haq, the UN deputy spokesman, quoted Ban as saying on Monday that the death toll was a result of missile attacks and that victims included children.
Ban called the attacks "blatant violations of international laws" that "are further degrading an already devastated healthcare system and preventing access to education in Syria".
The secretary-general said 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.
He did not say who was responsible for Monday's attacks, but some monitor groups suspect Russian raids.
In separate statements on Monday, France and Turkey branded the incident a "war crime".
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MSF Hospital hit
A hospital in the town of Maaret al-Numan in Idlib province, northwest Syria, supported by the medical charity Doctors Without Borders (MSF), was among those hit, the aid group said.
MSF said the strikes devastated the hospital and that at least seven people had been killed and at least eight more were missing, presumed dead.
"The author of the strike is clearly ... either the government or Russia," Mego Terzian, the group's France president, told the Reuters news agency.
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MSF said those killed in the strikes were five patients, a caretaker and a guard.
"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," MSF Syria operations chief Massimiliano Rebaudengo said.
In a separate incident, at least 14 people were killed and about 30 injured when air strikes and rocket artillery damaged parts of a hospital in the town of Azaz in Aleppo province, the media office at the rebel-controlled Aleppo local council said on Monday.
In the same raid, a school where refugees were sheltering was also hit.
The head of the media office, Abu Thaer al-Halabi, told Al Jazeera that a section of a highway that facilitates the main supply line for humanitarian aid to the region was destroyed in the raids.
Halabi said the strikes were carried out by Russian jets.
Also in Idlib province, another strike hit the National Hospital in Maarat Al-Nouman.
War of words
The Syrian government, backed by Russian air strikes, launched a major offensive from the north of Aleppo and captured several strategically important towns earlier this month.
The offensive has led to the displacement of more than 50,000 civilians from Aleppo, tens of thousands of whom have amassed in camps at the Turkish border.
Yet, the strikes have angered Turkey, with Ahmet Davutoglu, the Turkish prime minister, on Monday issuing a stark warning to Russia.
"If Russia continues behaving like a terrorist organisation and forcing civilians to flee, we will deliver an extremely decisive response," he said.
His comments escalated a war of words with Moscow, which earlier criticised Turkey's shelling of Kurdish-held positions in northern Syria as "provocative" and said that it wanted the issue raised at the Security Council.
Turkey continued shelling Kurdish-led forces in several parts of Aleppo province, alarmed by their recent advances against rebel groups in the country.
Ankara accuses the Kurdish forces of links to the Kurdistan Workers Party (PKK), an outlawed movement that has waged a decades-long armed campaign against the Turkish state. Turkey is concerned that the Kurds will be able to create a contiguous Kurdish territory just across the border in northern Syria.
The UK-based Syrian Observatory for Human Rights said at least two children were killed in Monday's shelling.
But despite the bombardment, the coalition of Kurdish and Arab fighters known as the Syrian Democratic Forces (SDF) was advancing inside the key rebel-held town of Tal Rifaat, the Observatory said.
The town, barely 20km from the Turkish border, is held by an alliance of rebels and is one of their few remaining bastions in the area.
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The Observatory said that SDF fighters now controlled more than 70 percent of the town, with heavy fighting continuing in its north and northeast.
The SDF has already seized the nearby Menagh airbase from rebel forces and severed the road between Tal Rifaat and Azaz.
Davutoglu warned on Monday that Ankara "will not let Azaz fall" to the SDF, adding "the necessary intervention will be made".
But an SDF spokesman told Al Jazeera on Monday: "Turkey artillery shelling has not and will not stop our advance to Azaz".
More than 260,000 people have been killed in Syria since the country's conflict began with anti-government protests in March 2011.
Source: Al Jazeera and agencies