Russia is facing intense criticism 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, capitalising 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 on 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.
UN convoys off to Syrian besieged zones
Meanwhile, the Syrian government has approved access to seven besieged areas and UN convoys are expected to set off in days, the United Nations says after crisis talks in Damascus.
UN Special Envoy Staffan de Mistura, who won the green light at talks with Foreign Minister Walid al-Moualem, said on Tuesday that the world body would test the government commitment to allow access on Wednesday but gave no details.
Their meeting in Damascus came at a time when government forces have been advancing rapidly with the aid of Russian air strikes, and just days before an internationally agreed pause in fighting is due to take effect.
De Mistura said they had discussed the issue of humanitarian access to areas besieged by all sides in the five-year war.
UNICEF staff has confirmed cases of severe malnutrition among children in Syrian town of Madaya. (AAP)
"It is clear it is the duty of the government of Syria to want to reach every Syrian person wherever they are and allow the UN to bring humanitarian aid," de Mistura said in a statement. "Tomorrow we test this."
The UN Office for the Coordination of Humanitarian Affairs (OCHA) said that Syria had approved access to Deir al-Zor; Foua and Kafraya in Idlib; and Madaya, Zabadani, Kafr Batna and Mouadamiya al-Sham in rural Damascus.
"Humanitarian agencies and partners are preparing convoys for these areas, to depart as soon as possible in the coming days," the OCHA said.
It was not immediately clear whether the convoys would begin on Wednesday, as de Mistura had indicated.
Nor was there any indication of a breakthrough on access to areas besieged by armed opposition groups.
UN-backed peace talks are scheduled to resume in Geneva on February 25, after de Mistura suspended a first round earlier this month.
Last Friday global powers meeting in Munich agreed to the pause in fighting in the hope that this could allow the talks to resume, but the deal does not take effect until the end of this week and was not signed by the Syrian warring parties.
Syria's ambassador to the UN in Geneva, Hussam Aala, said in an interview in the daily Tribune de Geneve: "We have done all we could to facilitate the passage of aid convoys in January and February."
"The advance of the Syrian army in this region has allowed us to break the siege imposed against two towns, Nubul and al-Zahra. It opened the way for the Syrian Arab Red Crescent to deliver aid to 70,000 residents. Our objective was to cut all the supply routes for arms and for men to the terrorist groups armed by Turkey."
