Russia on Tuesday said it "categorically rejects" accusations of war crimes over the bombing of hospitals in Syria.
"Those who make such statements are not capable of backing them up with proof," Kremlin spokesman Dmitry Peskov said.
Up to 50 people were killed in missile attacks on at least four hospitals and a school in rebel-held areas of northern Syria on Monday.
The UN said "intentionally directing attacks" at hospitals and medical units would constitute a war crime.
Russia has been accused, by Turkey, among others, of being responsible for the attacks, BBC reported.
Monday's strikes hit two hospitals -- including one for mothers and babies -- and a school sheltering internally displaced people in Azaz, near the border with Turkey, the UN said. Thirty-four people were killed and dozens were injured.
Two hospitals were also struck in Maarat al-Numa, Idlib province, killing at least 12 people and wounding about 36.
One of the hospitals in Maarat al-Numan was supported by Medecins Sans Frontieres (MSF). It was reportedly struck by four missiles in what MSF said was "direct targeting" over the course of 90 minutes.
Peskov said the only proof Russia would accept from the ground "comes from the Syrian authorities". He said their evidence "shows the opposite".
The Syrian ambassador to Moscow, Riad Haddad, previously said the US was to blame, a claim the Pentagon dismissed as "patently false".
International humanitarian law bans any attack on patients and medical personnel or indeed any attack on medical facilities, unless they are used for military purposes.
However, even if they are identified as a military objective, such a target must not be attacked if the scale of collateral civilian casualties is likely to be disproportionate to the military gain.
The strikes came days after world powers -- including Russia -- agreed to work towards a selective truce in Syria, due to begin later this week.
The UN envoy to Syria, Staffan de Mistura, met Syrian Foreign Minister Walid Muallem on Tuesday, and planned to meet him again later in the day.
They were due to discuss among other things one of the key priorities of the truce -- "unhindered humanitarian access to all besieged areas".
There is no word yet on when aid convoys might reach those areas.
Earlier, Syrian President Bashar al-Assad, in his first comments on Friday's "cessation of hostilities" plan, said it was doubtful all parties would be putting down their weapons within a week.
A Turkish official on Tuesday said Turkey would back a ground operation in Syria but only "with our international allies".
"There is not going to be a unilateral military operation from Turkey to Syria," an official said in Istanbul.
Turkey has been watching with growing alarm the fighting on the Syrian side of its borders -- accusing Russian warplanes of violating its airspace and warning against recent Kurdish successes.
Meanwhile, Syrian government forces -- backed by Russian air power -- reportedly continue to make advances around the northern city of Aleppo, capturing the villages of Ahras and Misqan on Tuesday.
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.
