Moscow on Monday insisted its bombing campaign in Syria was "highly effective", denying its jets struck hospitals as a brutal onslaught against rebel-held Aleppo has sparked accusations of war crimes.

Deputy foreign minister Gennady Gatilov said Russia's air strikes in support of President Bashar al-Assad's regime had stopped extremists taking over the country.

"From this perspective our participation has been highly effective, especially now when the situation around Aleppo has worsened," he was quoted as saying by Russian wires.

Moscow has been accused of indiscriminately bombing Aleppo's opposition-controlled east as it helps an offensive currently being conducted by Syrian government troops to capture all of the country's second city.

The largest hospital in eastern Aleppo was bombed over the weekend for the second time in days as the UN's top aid official decried the "living hell" suffered by locals.

Russia has repeatedly denied that its strikes have hit hospitals and other civilian facilities.

"Accusations that Russia is allegedly bombing medical facilities, hospitals, schools are all baseless," Gatilov said.

A short-lived truce brokered by Moscow and Washington earlier this month could have led the two countries to coordinate strikes against extremists, but the deal quickly fell apart.

The United Nations has warned that a humanitarian catastrophe is unfolding in Aleppo unlike any witnessed so far in Syria's brutal five-year war, which has claimed more than 300,000 lives.

Moscow said last week that it was continuing its year-long Syrian air campaign in spite of US warnings that Washington would suspend talks unless Moscow stopped its assault on Aleppo.

Gatilov said Russia was working to agree with Washington on "what needs to be done to renew the cessation of hostilities."