Syrian pro-government forces used intense air strikes as cover for an advance in the battleground city of Aleppo on Friday, ahead of fresh diplomatic efforts to end the country's intractable conflict.
The United States and Russia, which support opposite sides in the five-year war, will meet in Switzerland on Saturday to try to resurrect the peace process.
Moscow has faced rising international criticism over its backing for President Bashar 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 Syrian Observatory for Human Rights, a British-based monitor, said Russian and Syrian warplanes pounded opposition-controlled eastern districts again on Friday, 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.