Russia faced intense criticism Tuesday for its aerial campaign backing the Syrian government after it was accused of hitting hospitals in opposition-held areas.
Turkey and France both said Russia had committed war crimes, while UN human rights spokesman Rupert Colville said it appeared that the bombings were a military tactic and could amount to war crimes.
Russia formally denied it hit a Doctors Without Borders (MSF) hospital in northern Syria, an area where rebels are ceding ground to both government forces and the Kurdish YPG militia, capitalizing on Moscow's airstrikes.
"We categorically denounce such claims. Every time it turns out that whoever makes such claims is unable to even somehow confirm them," Kremlin spokesman Dmitry Peskov said in comments carried by state news agency TASS.
Almost 50 people, including many children, were killed Monday in multiple missile attacks on hospitals and schools in Syria, the UN said, without specifying who was responsible.
An updated UN statement on Tuesday said a total of seven hospitals were out of service as a result of the attacks, while 13 hospitals had been reported hit in the previous month.
An expert on the conflict said that he was in no doubt that Russia was responsible for the attacks, saying they were "designed" to empty hostile regions of both civilians and fighters.
"It's a well-known tactic: you destroy hospitals so that you let people know if they are injured they won't be treated," Thomas Pierret, director of the University of Edinburgh's Centre for the Advanced Study of the Arab World, told dpa.
"And any fighters are also under serious Russian pressure because they know if they are injured in any way they won’t be treated and they will die from their wounds," he said.
The Russian Defence Ministry said its military had hit 1,600 "terrorist facilities" in Syria but did not confirm any strikes on civilians.
Meanwhile, UN special envoy for Syria Staffan de Mistura was in Damascus, trying to give life to efforts to reach a ceasefire backed by world powers and slated to take effect on Friday.
De Mistura said he had discussed humanitarian access to besieged areas with Syrian Foreign Minister Walid al-Moallem.
"Tomorrow we will test this, and we will be able to talk more about it," the UN envoy said.
The Syrian government has approved humanitarian access to seven besieged areas - Deir ez-Zor, Foah and Kafraya in Idleb, and Madaya, Zabadani, Kafr Batna and Madamiyet Elsham in rural Damascus, said Farhan Haq, a spokesman for UN Secretary General Ban Ki-moon.
Humanitarian agencies are preparing convoys to depart to these areas as soon as possible, Haq said in New York.
As de Mistura arrived, Syrian President Bashar al-Assad poured cold water on the ceasefire proposal, raising worries that the government, which is on a winning streak against rebels, will not be keen on a deal.
De Mistura has called for the peace talks to resume February 25 in Geneva.
Syrian rebels are losing ground in the north under intense Russian airstrikes, and Kurdish forces have seized a string of villages near the Turkish border, alarming Ankara.
Turkish artillery bombarded Kurdish positions around the border city of Azaz overnight, killing at least three Syrian Kurdish YPG fighters, according to the Syrian Observatory for Human Rights, a monitoring group.
The Kurds are moving farther east, pushing toward the rebel stronghold Marea, which borders territory held by Islamic State.
The Dogan news agency in Turkey said Turkish military sources claimed the shelling was in response to Kurdish fire toward Turkey, which the Kurds denied.
Prime Minister Ahmet Davutoglu says Turkey's "national security" is threatened by the advance of the Kurds and the collapse of the rebels, and has pledged that Azaz would not fall out of opposition control.
Ankara is worried about the Syrian Kurds' links to the banned Kurdistan Workers' Party (PKK) which is fighting state security forces.
The United States urged the YPG to halt their advances.
"The YPG needs to stop its own actions on the ground that we believe raises tensions, but we would also urge Turkey to cease firing artillery across the border. We just think it escalates tensions in the region," State Department spokesman Mark Toner said in Washington.
In New York, members of the UN Security Council expressed concern during a closed meeting over Turkey's attack in Syria, said Rafael Carreno, Venezuelan ambassador to the UN and current president of the council.
"All members of the UN Security Council have agreed to ask Turkey to comply with international law," Carreno said.