Syrian pro-government forces used intense air strikes as cover for an advance in the battleground city of Aleppo ahead of fresh diplomatic talks on today to end the conflict.
The United States and Russia, which support opposite sides in the five-year war, will meet in Switzerland to try to resurrect the peace process.
Moscow has faced rising international criticism over its backing for President Bashar al-Assad's onslaught in divided Aleppo, including Western accusations of possible war crimes.
Violence has continued unabated in the northern city, once Syria's commercial hub but now ravaged by Russian and regime air strikes in support of a major government offensive against rebels.
The meeting comes after leading charities called today for a ceasefire in the battered city, issuing a joint plea "to establish a ceasefire of at least 72 hours in east Aleppo".
"This will allow the sick and wounded to be evacuated, and for food and medical aid to enter the besieged area," said a statement from Save the Children, which joined the International Rescue Committee, the Norwegian Refugee Council and Oxfam International in calling for a truce.
The Syrian Observatory for Human Rights, a British-based monitor, said Russian and Syrian warplanes pounded opposition-controlled eastern districts again yesterday, though it did not have any immediate information on casualties.
It said pro-government forces had used the air raids to advance southwards from positions in north Aleppo with the goal of "opening a route to the airport", east of the city.
The intensified bombardment has put a severe strain on rescue workers and medical staff in east Aleppo, home to an estimated 250,000 residents under siege.
"This recent escalation has been huge and we've had a lot of work," said Ibrahim Abu al-Leith, a spokesman for the White Helmets rescue force in Aleppo.
He said rescuers were still working to dislodge people from under the rubble in the Tariq al-Bab eastern district.
AFP's correspondent in east Aleppo said some people had been stuck under the rubble for at least two days as rescuers scrambled between neighbourhoods.
Others bled to death after White Helmets teams were unable to reach them in time.
Since the collapse last month of a truce brokered by Washington and Moscow, Aleppo has been engulfed by some of the worst violence of the conflict.
More than 370 people, including nearly 70 children, have been killed in regime and Russian bombardment of east Aleppo since the regime's assault began on September 22, the Observatory said.
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