The US secretary of state has called on the Syrian government and its Russian backers to “show a little grace” as their forces close in on the remaining rebel-held area of the city of Aleppo.
John Kerry was speaking after a meeting in Paris of governments that back the Syrian opposition.
Latest reports from the city say the rebel enclave is being hit by repeated air and rocket strikes.
Civilians are said to be streaming out of the city in large numbers.
The Paris meeting on Syria was attended by officials from the US, Europe and some Arab countries.
They renewed calls for the Syrian government and Russia, whose forces are backing the government of President Bashar al-Assad, to allow people to leave Aleppo.
Russia says the exodus from the rebel-held eastern parts of Aleppo is growing amid the push by the Syrian army to regain control of the whole city.
About 50,000 civilians have fled the rebel enclave over the past two days, a Russian defence spokesman said.
He added that more than 1,000 rebels had laid down their arms as pro-government forces closed in.
Mr Kerry said: “Russia and Assad have a moment where they are in a dominant position to show a little grace.
“I believe there could be a way forward but it depends on big, magnanimous choices from Russia.”
His British counterpart, Boris Johnson, said: “Although the deterioration of the situation in Aleppo will be a setback for the Syrian opposition, it will not change the fundamentals of the conflict.
“There can be no military solution in Syria. We must keep pushing for a return to a political process with the credibility necessary for all parties to commit to an end to all the fighting.”
The BBC’s Hugh Schofield in Paris says the language suggests the US government and its allies are resigned to an imminent Syrian government victory in Aleppo, and can do little more than plead for it to allow safe passage out of the city for opposition fighters.
The Russian military says the Syrian government now controls 93% of Aleppo.
The intensification in fighting in recent weeks has forced tens of thousands to seek refuge in government-controlled territory.
On Friday the UN estimated that up to 100,000 people had been squeezed into an “ever-shrinking” rebel pocket in eastern Aleppo with little or no access to food or medical care.
Syria’s government has said it is ready to resume dialogue with the opposition but without “external intervention or preconditions”.
Aleppo was once Syria’s largest city and its commercial and industrial hub before the uprising against President Bashar al-Assad began in 2011.
It has been divided roughly in two since mid-2012. But in the past year, Syrian troops broke the deadlock with the help of Iranian-backed militias and Russian air strikes.
Elsewhere in Syria, militants from the so-called Islamic State group have been advancing on Syrian government positions in the countryside around the ancient city of Palmyra.