Russia has announced it will veto a French-drafted UN resolution calling for a ceasefire in the besieged Syrian city of Aleppo.

Russia's UN Ambassador Vitaly Churkin said the Security Council should instead rally around the proposal made by UN envoy Staffan de Mistura for an al Qaida-linked militant faction to leave Aleppo in exchange for a halt to Russian and Syrian government bombardment.

He told reporters after Mr de Mistura briefed the council on Friday behind closed doors that "the French proposal is very hastily put together, and I frankly believe that this is designed not to make progress" in ending the current stalemate "but to cause a Russian veto".

Mr Churkin said it was "unprecedented" that the 15-member council would ask one of the five permanent members to limit its activities, in this case requiring the Russian military to stop flights.

Asked if Russia will veto the French draft, Mr Churkin said he never uses the word until he gets instructions from Moscow, but "I cannot possibly see how we can let this resolution pass".

France's UN Ambassador Francois Delattre insisted that a vote on its resolution, co-sponsored by Spain, will go ahead on Saturday.

Earlier US secretary of state John Kerry said Russia and Syria should face a war crimes investigation for their attacks on Syrian civilians.

Mr Kerry said Syrian forces hit a hospital overnight, killing 20 people and wounding 100, in what would be the latest strike by Moscow or its ally in Damascus on a civilian target.

"Russia and the regime owe the world more than an explanation about why they keep hitting hospitals, and medical facilities, and women and children," Mr Kerry told reporters alongside French foreign minister Jean-Marc Ayrault, who arrived in Washington directly from meeting Russian officials in Moscow.

Mr Kerry said such acts "beg for an appropriate investigation of war crimes".

"They are beyond the accidental now, way beyond," he added in some of his toughest criticism to date. He accused Russia and Syria of undertaking a targeted strategy to "terrorise civilians".

Mr Kerry's September 9 agreement with Russian foreign minister Sergey Lavrov would have created a new counter-terrorism alliance in Syria, if fighting had stopped for a week and aid deliveries been permitted to reach desperate civilians in rebel-held parts of Aleppo and other besieged areas.

Neither condition was met.

The truce then broke completely when Syria and Russia renewed their military offensive in Aleppo. Mr Kerry ended bilateral discussions with Moscow on the military partnership earlier this week.

The war has killed as many as half a million people since 2011, contributed to Europe's worst refugee crisis since the Second World War and allowed Islamic State to carve out territory for itself and emerge as a global terror threat.

As Mr Kerry and Mr Ayrault spoke, Russia's lower house of parliament ratified a treaty with Syria that allows the Russian military to stay indefinitely in the Middle Eastern country.

The move comes as a show of support for embattled Syrian president Bashar Assad, and allows Russia to use the base free of charge and for as long as it requires.