WASHINGTON (Reuters) - 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 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.

Russian or Syrian warplanes knocked a major Aleppo hospital out of service, hospital workers said, and ground forces stepped up an assault on the city’s besieged rebel sector in a battle that may become a turning point in the 5-1/2 year civil war. Syria and Russia say they target only militants of Islamic State and other groups.

The Sept. 19 collapse of a U.S. and Russian-brokered truce and the dramatic intensification of the Syrian government’s offensive on Aleppo has forced U.S. officials to reconsider what, if anything, they might do to try to end the violence.

So far, U.S. President Barack Obama has refused to commit U.S. forces to try ousting Syrian President Bashar al-Assad and U.S. officials doubt he would do so in his final months in office. A more plausible scenario would be to allow Gulf Arab allies to give the rebels more weapons.

“If the diplomacy stops, other options will have to be looked at, which means discussions of things like ... more aggressive measures,” said a U.S. official, who spoke on condition of anonymity.

If the government took full control of Aleppo, it would restore its rule over western Syria’s most important cities, where nearly all of the population lived before the start of the war that has since made half of Syrians homeless, caused a refugee crisis and contributed to the rise of Islamic State.

HOLDING RUSSIA ACCOUNTABLE

In a statement on Kerry’s conversation with Lavrov, the U.S. State Department said it might halt a diplomatic process that already seems moribund and expressed concern over “continued‎ Russian and Syrian regime attacks on hospitals, the water supply network, and other civilian infrastructure in Aleppo.”

“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,” State Department spokesman John Kirby added.

Kerry also told Lavrov that Washington expects Russia to take “immediate steps to end the assault on Aleppo” and restore the 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 International Criminal Court does not automatically have jurisdiction because Syria is not a party to the treaty that set up the Hague-based court. A U.N. Security Council resolution to refer the matter to the ICC is a virtual impossibility because Russia, which has a veto on the council, is sure to block this.

French Foreign Minister Jean-Marc Ayrault said he was working to put forward a U.N. Security Council resolution for a ceasefire in Aleppo, saying any country that opposed it could be complicit in war crimes.