Overwhelmed rescue workers combed rubble for victims of intense air strikes on Syria's battleground city 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 swelling international criticism over its backing for President Bashar al-Assad's onslaught in 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.
"Very intense air raids targeted several neighbourhoods from dawn until mid-morning" on Friday, said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, a British-based monitor.
He said both Russian and Syrian warplanes were involved but had no immediate word on casualties.
The intensified bombardment has put a severe strain on rescue workers and medical staff in eastern 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.
"The civil defence team hasn't slept in four days because of the bombardment on the eastern neighbourhoods. Even our machines are exhausted," he told AFP.
He said rescuers were still working to dislodge people from under the rubble in the Tariq al-Bab eastern district.
- New truce talks -
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.
Just days after the deal fell apart, Syria's military announced an offensive to capture the whole city, divided between rebels in the east and the government in the west since 2012.
More than 370 people, including nearly 70 children, have been killed in regime and Russian bombardment of east Aleppo since the assault began, the Observatory said.
Dozens of civilians, including children, have also died in rebel bombardment of regime-controlled western districts, according to the monitor, which compiles its information from sources on the ground.
US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov are expected to hold fresh talks to try to revive the ceasefire deal in Lausanne on Saturday.
UN Syria envoy Staffan de Mistura will attend, along with the chief diplomats from Turkey, Saudi Arabia and Qatar -- all backers of Syrian opposition forces.
Then in London on Sunday, Kerry will likely meet up with his counterparts from Britain, France and Germany.
Lavrov has said he hopes discussions in Switzerland could help "launch a serious dialogue" based on the now-defunct US-Russian pact.
He also spoke Thursday with his French counterpart Jean-Marc Ayrault, despite tensions between the countries over Syria that prompted President Vladimir Putin to cancel a trip to Paris.
The French foreign ministry said Ayrault stressed "the urgency to break the current impasse... and achieve a halt in the bombing in Aleppo".
- Aleppo as a 'springboard' -
In an interview with Russia's Komsomolskaya Pravda tabloid published Friday, Assad said he would use a victory in Aleppo as a "springboard" to capture other rebel strongholds.
"It's going to be the springboard, as a big city, to move to another areas, to liberate another areas from the terrorists," he said.
He indicated that his next target could be northwestern Idlib province, held by an alliance of rebels and jihadists including the Fateh al-Sham Front, which changed its name from Al-Nusra Front when it cut ties with Al-Qaeda.
"You have to keep cleaning this area and to push the terrorists to Turkey to go back to where they come from, or to kill them. There's no other option," Assad said.
Putin on Friday approved a law ratifying his deal with Syria, first signed in August 2015, to establish Russia's Hmeimim airbase to launch pro-regime operations.
The official signing off makes the deal valid for an "indefinite" period and is a legal move that many believe will pave the way for Moscow to make the base permanent.
Russia has long provided political and financial support to Syria. It began its own bombing campaign against anti-government groups there in September 2015.
Moscow offered on Thursday to "ensure the safe withdrawal" of rebel groups and civilians from eastern Aleppo. There was no immediate response from the rebels.
Near Damascus, more than 1,200 people including rebel fighters and their families were bussed out of the towns of Qudsaya and al-Haamah on Thursday under a local deal with the government.
They were transported northwest to Idlib province.
