Aleppo (Syria) (AFP) - 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 Saturday 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 Saturday 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 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.

He said rescuers were still working to dislodge people from under the rubble in the Tariq al-Bab eastern district.

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- Bleeding to death -

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.

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.

The Observatory said at least 15 civilians, including two children, were killed Friday in air raids on a village in northeastern Raqa province, "presumably by the US-led coalition" currently battling the Islamic State jihadist group in Syria and Iraq.

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.

Iran announced late Friday that its foreign minister Mohammad Javid Zarif would take part in the Lausanne talks.

Then in London on Sunday, Kerry will likely meet up with his counterparts from Britain, France and Germany.

Lavrov played down hopes of a breakthrough in Lausanne, telling reporters on Friday he had no "special expectations" for progress.

- 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," the Syrian president said.

He said his next target could be northwestern Idlib province.

Idlib is 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.

Also Friday, Russian President Vladimir Putin approved a law ratifying a deal with Syria -- first signed in August 2015 -- to establish Russia's Hmeimim airbase to launch pro-regime operations.

Russia has long provided political and financial support to Syria and began its bombing campaign there in September 2015.