The United States threatened on Wednesday to halt its diplomacy with Russia on Syria and it said it held Russia accountable for the use of incendiary and bunker-busting bombs on Aleppo - a view a U.S. official said could open Moscow up to charges of war crimes.

Secretary of State John Kerry conveyed the message in a call to Russian Foreign Minister Sergei Lavrov in which he voiced "grave concern" over the Russian-backed Syrian government's air and land assault on rebel areas of Aleppo, Syria's largest city.

"Secretary Kerry expressed grave concern over the deteriorating situation in Syria, particularly for continued Russian and Syrian regime attacks on hospitals, the water supply network, and other civilian infrastructure in Aleppo," State Department spokesman John Kirby said in a written statement.

"The Secretary made clear the United States and its partners hold Russia responsible for this situation, including the use of incendiary and bunker buster bombs in an urban environment, a drastic escalation that puts civilians at great risk," he added.

Kerry also told Lavrov Washington was preparing to end its diplomatic engagement with Moscow on Syria - including possible intelligence-sharing - if Russia did not take "immediate steps to end the assault on Aleppo" and restore a defunct ceasefire.

A U.S. official said that the use of incendiary and bunker busting weapons in an area with civilians could open Russia up to accusations of war crimes but said there is no forum in which Russia might realistically face such charges.

"The use of such weapons in a populated area in a place civilians are known to be present could be viewed as an indiscriminate attack and thus a war crime," said the official.

Meanwhile, France's foreign minister said on Wednesday he was working to put forward a UN Security Council resolution for a ceasefire in the Syrian city of Aleppo, and that any country that opposed it would be deemed complicit in war crimes.

Speaking to lawmakers, Jean-Marc Ayrault accused Syria, backed by Russia and Iran, of carrying out an "all-out war" on its people, something that Paris could not sit by idly watching.

"At this very moment, we are proposing to discuss a resolution to obtain a ceasefire in Aleppo," Ayrault said. "This resolution will leave everyone facing their responsibilities: those who don't vote for it, risk being held responsible for complicity in war crimes."

Since the collapse of a ceasefire brokered by Washington and Moscow, Russian and Syrian warplanes have launched their biggest offensive on Aleppo's besieged rebel-held sectors, in a battle that has become a potentially decisive turning point in the five-year civil war.

With Western states reluctant to pour weapons into Syria to change the military balance of power, Paris hopes that by taking the matter directly to the United Nations Security Council it may be able to force Moscow into some form of compromise.

"We will not let Aleppo become the Guernica of the 21st century," Ayrault said, referring to the indiscriminate bombing of civilians in the Basque town during the Spanish Civil War of the 1930s.

A senior French diplomatic source said Paris had prepared a text that it wanted to discuss with Britain and the United States before presenting it to the Russians.

"The idea is not to get a Russian veto, but to have a proper discussion with them on ending the violence, enabling humanitarian access and putting a mechanism in place to enforce the ceasefire," said the diplomat.

"If they don't play the game then we will have no qualms taking this to the Security Council even if it means a Russian veto."

Britain's deputy UN Ambassador Peter Wilson said diplomats were looking to "move on this as quickly as we possibly can".

A senior council diplomat, speaking on the condition of anonymity, said it was time to force Moscow's hand. "We're trying to embarrass Russia into doing the right thing on the ground," he said. "Our priority is action on the ground ... If that doesn't work then I think we're back in the Security Council."

France has also been pushing for a separate Security Council resolution after a joint investigation by the UN and the global chemical weapons watchdog OPCW found that Syrian government troops were responsible for two toxic gas attacks and Islamic State militants used sulphur mustard gas.

"The Security Council must condemn the use of chemical weapons under chapter 7," Ayrault said.

Chapter 7 deals with sanctions and authorisation of military force by the Security Council.-Reuters