Moscow and Damascus launched a campaign to recapture the rebel-held sector of Syria’s biggest city this month, abandoning a ceasefire a week after it took effect to embark on what could be the biggest battle of a nearly six-year war.
Syrian government forces made a significant advance, capturing the Handarat refugee camp a few kilometres north of the city.
They had briefly seized it on Saturday, before losing it again in a rebel counter attack.
Rebel fighters have launched an advance of their own near the central city of Hama, where they said they made gains on Thursday.
The United States and European Union accuse Russia of torpedoing diplomacy to pursue military victory in Aleppo, and say Moscow and Damascus are targeting civilians, hospitals and aid workers to break the will of 250,000 people living under siege in the city.
Syria’s UN Ambassador Bashar Ja’afari rejected accusations that his government was killing civilians.
But US officials are searching for a tougher response to Russia’s decision to ignore the peace process and seek military victory on behalf of President Bashar al-Assad.
“We are on the verge of suspending the discussion because it is irrational in the context of the kind of bombing taking place to be sitting there trying to take things seriously,” Kerry told an audience in Washington.
“It is one of those moments where we are going to have to pursue other alternatives,” he added.
Recapturing Aleppo would be the biggest victory of the war for government forces, and a potential turning point in a conflict that until now most outside countries had said would never be won by force.
The multi-sided civil war has killed hundreds of thousands of people, made half the Syrian population homeless, and allowed much of the east of the country to fall into the hands of Islamic State jihadists who are enemies of all other sides.
EU foreign affairs chief Federica Mogherini called the air strikes in Aleppo a “massacre” and said European governments were considering their response. Russia and the Syrian government say they are targeting only militants.
Kremlin spokesman Dmitry Peskov, meanwhile, said Russia would “continue the operation of its air force in support of the anti-terrorist activity of Syria’s armed forces”.