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 two schools in rebel-held areas of northern Syria on Monday.

The UN said the "deliberate" targeting of such facilities "could amount to war crimes", according to Reuters.

Russia has been accused, by Turkey among others, of being responsible for the attacks.

Meanwhile, 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.

Ceasefire doubts

Monday's strikes hit two hospitals and two schools in Azaz, near the border with Turkey, and at least two hospitals in Maarat al-Numan, further south.

One of the hospitals in Maarat al-Numan was run by Medecins Sans Frontieres (MSF), which called the attack a "deliberate" strike.

Mego Terzian, president of MSF France, told Reuters "either the [Syrian] government or Russia" was responsible.

But 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".

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 was planning 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 cast doubt over the "cessation of hostilities" plan, saying it did not mean all the parties would put down their weapons.

"So far they say they want a ceasefire within a week," he said in his first comments of Friday's agreement. "Who is capable of gathering all these conditions and requirements within a week?"

Syrian government forces - backed by Russian air power - are reportedly continuing to make advances around the northern city of Aleppo, capturing the villages of Ahras and Misqan on Tuesday.

Separately, the US-backed Kurdish-led Syrian Democratic Forces (SDF) were reported to have captured Tal Rifaat, a key town in Aleppo province, and had "encircled" armed Islamist militants in the town of Mari.

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.