Washington: Obama administration officials have begun considering tougher responses to the Russian-backed Syrian government assault on Aleppo, including military options, as rising tensions with Moscow diminish hopes for diplomatic solutions from the Middle East to Ukraine and cyberspace, US officials said on Wednesday.

The discussions were being held at "staff level," and have yet to produce any recommendations to President Barack Obama, who has resisted ordering military action against Syrian President Bashar al-Assad in the country's multisided civil war.

But the deliberations coincide with Secretary of State John Kerry threatening to halt diplomacy with Russia on Syria and holding Moscow responsible for dropping incendiary bombs on rebel areas of Aleppo, Syria's largest city. It was the stiffest US warning to the Russians since the September 19 collapse of a truce they jointly brokered.

Administration advocates of a more muscular US response said on Wednesday that it was not clear what, if anything, the president would do, and that his options "begin at tougher talk," as one official put it. One official said that before any action could be taken, Washington would first have "follow through on Kerry's threat and break off talks with the Russians" on Syria.