Russia says it "categorically rejects" accusations of war crimes over the bombing of hospitals in Syria.
Kremlin spokesman Dmitry Peskov said "those who make such statements are not capable of backing them up with proof".
Up to 50 people were killed in missile attacks on at least four hospitals and a school in northern Syria on Monday.
Meanwhile the UN's Syria envoy has said the Syrian government has a duty to allow aid deliveries - and that this would be tested on Wednesday.
Staffan de Mistura has been meeting Syrian Foreign Minister Walid Muallem in the capital Damascus.
"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." he said in statement. "Tomorrow we test this."
Russia has been accused, by Turkey among others, of being responsible for the attacks on rebel-held areas.
The UN said "intentionally directing attacks" at hospitals and medical units would constitute a war crime.
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 wounded.
Two hospitals were also struck in Maarat al-Numan, further south in 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.
MSF said 11 people died, including five patients, and that the toll was likely to rise.
Mego Terzian, president of MSF France, told Reuters "either the [Syrian] government or Russia" was responsible.
But Mr Peskov told the BBC that 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".
What does the law say about bombing hospitals?
- 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
- Indiscriminate attacks are prohibited
The strikes came days after world powers - including Russia - agreed to work towards a selective truce in Syria, due to begin later this week.
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.
Turkey backs ground offensive
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," the unnamed official told reporters 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.
Turkey views the Kurdish YPG militia as allied to the outlawed PKK, which has carried out a decades-long campaign for autonomy in Turkey.
The YPG is part of the US-backed Syrian Democratic Forces (SDF) which captured Tal Rifaat, a key town in Aleppo province, and is headed towards Azaz.
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.