Russia has said it will press on with its bombing campaign in Syria despite international criticism
Russia has said it will press on with its bombing campaign in Syria, as the United Nations pleaded for medical evacuations from war-ravaged Aleppo.
The developments came as former UN secretary general Kofi Annan said that "proxy wars" in Syria could prolong the five-year-old conflict.
Moscow is supporting a ferocious assault by the forces of Syrian President Bashar al Assad on the rebel-held eastern part of the city.
There have been calls for it to stop bombing Aleppo and join efforts to restore the shattered truce.
But the calls were rejected by Kremlin spokesman Dmitry Peskov, who blamed the US for the latest surge in violence.
He said Russia would "continue the operation of its air force in support of the anti-terrorist activity of Syria's armed forces".
US President Barack Obama and German Chancellor Angela Merkel have condemned the Russian and Syrian airstrikes as "barbarous".
"The President and Chancellor strongly condemned the barbarous Russian and Syrian regime airstrikes against eastern Aleppo, an area populated with hundreds of thousands of civilians, half of whom are children," a White House statement said.
"They agreed Russia and the Syrian regime bear special responsibility for ending the fighting in Syria and granting the UN humanitarian access to besieged and hard to reach areas in Syria."
Meanwhile, Mr Annan told Sky News: "The war has been a failure and what has been happening is outraging, and it is the Syrian people who are paying the price.
"There are lots of forces at work. There are proxy wars going on. And you sometimes have the feeling that some are determined to fight until the last Syrian life."
He was speaking after the UN's aid chief warned that Aleppo faces a humanitarian catastrophe "unlike any" witnessed so far in the fighting.
"Let me be clear: east Aleppo this minute is not at the edge of the precipice," Stephen O'Brien told the Security Council.
"It is well into its terrible descent into the pitiless and merciless abyss of a humanitarian catastrophe unlike any we have witnessed in Syria."
US ambassador Samantha Power described the past week as "the most savage we've seen in an incredibly savage five-plus-year war".
In the latest offensive more than 1,000 people have been killed by 1,700 air strikes on east Aleppo alone.
Ms Power said it was "soul-shattering" and accused Russia and President Assad of "unleashing a savagery" against civilians.
"We're at a turning point," she said.
Mr O'Brien appealed for action from the Security Council after diplomatic efforts at week's General Assembly meeting ended in failure.
"This revolting situation in Aleppo must, please, be the SOS, the May Day call, to the international community," he said.
In a sign of how desperate the situation has become, the UN has warned that "probably hundreds" of people needed to be evacuated from Aleppo for medical reasons.
The organisation's deputy envoy for Syria, Ramzy Ezzeldin Ramzy, said: "The bombing must stop. Civilians must be protected. And the cessation of hostilities must be restored."
His comments came a day after two of the largest hospitals in east Aleppo were bombed.
The present UN Secretary, General Ban Ki-moon, has described the attack as a war crime.