Top diplomats were to meet in Switzerland Saturday in a fresh push to revive a ceasefire in Syria, after pro-government forces intensified their bombardment of the battered city of Aleppo.

Since the collapse of a truce last month, Aleppo has been ravaged by a major onslaught by forces loyal to Syrian President Bashar al-Assad, backed by Russia, which has prompted renewed efforts to secure a pause in the fighting.

With Washington and Moscow backing opposite sides in the five-year war, the talks in Lausanne will bring together US Secretary of State John Kerry and his Russian counterpart Sergei Lavrov with top diplomats from the UN and regional powers including Iran.

Kerry plans to hold one-to-one meetings with Saudi Foreign Minister Adel al-Jubeir and then with Lavrov before the multilateral talks start later in the afternoon, a US official said.

Both sides have played down hopes of a breakthrough, with Lavrov telling reporters on Friday he had no “special expectations” of progress, while a senior official in Kerry’s entourage said the aim was to explore ideas for ending the fighting.

The talks come as Moscow faces growing criticism over its backing for Assad’s assault in divided Aleppo, prompting Western allegations of possible war crimes.

Intense air strikes hit rebel-held districts of east Aleppo again on Saturday, according to the Syrian Observatory for Human Rights, a British-based monitoring group with a network of sources on the ground.

Leading charities issued 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, the International Rescue Committee, the Norwegian Refugee Council and Oxfam International.

– Seeking concrete steps –

Aleppo has been engulfed by some of the worst violence of the conflict since the collapse of last month’s truce deal, which was brokered by Washington and Moscow and which diplomats are now hoping to revive.

“I think we need to see what happens in the room to determine whether this is the beginning of a new process that continues in this format or not,” said a senior US official travelling with Kerry, who arrived in Lausanne at around 0900 GMT.

Lavrov on Friday insisted that Russia was not planning on presenting new initiatives on ways to resolve the conflict, which has claimed more than 300,000 lives since it erupted in 2011.

Instead he said Moscow would call for “concrete steps” to implement earlier UN resolutions and the now defunct US-Russia ceasefire deal.

Speaking to AFP, a French diplomatic source said he did not hold out much hope.

“When you see the results from the previous efforts, quite frankly I’m a bit sceptical about the next ones,” he said.

Some believe Russia could be playing for time in a bid to solidify its positions ahead of the US presidential elections, now only weeks away.

“The Russians are seeking to maximise their advantage before (President Barack) Obama’s successor — probably (Hillary) Clinton — steps in with a likely firmer approach to Syria,” said Karim Bitar, a researcher at the Institute for International and Strategic Affairs think tank in Paris.

Kerry and Lavrov will be joined by UN Syria envoy Staffan de Mistura, as well as top diplomats from Turkey, Saudi Arabia and Qatar — all backers of Syrian opposition forces.

Iran, a key Assad supporter, said late Friday that Foreign Minister Mohammad Javad Zarif will also take part. Egypt, Iraq and Jordan will also be represented, the US official said.

Kerry is then due to head to London, where he is expected to meet Sunday with his counterparts from Britain, France and Germany.

– Huge escalation –

The intensified bombardment has put increasing strain on rescue workers in besieged eastern Aleppo which is home to an estimated 250,000 residents.

“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.

More than 370 people, including nearly 70 children, have been killed in regime and Russian bombardment of eastern Aleppo since the assault began on September 22, according to the Observatory.

Dozens of civilians, including children, have also died in rebel bombardment of regime-controlled western districts, the monitor said.

Buoyed by his forces’ gains in Aleppo, Assad said he would use a victory there 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 told Russia’s Komsomolskaya Pravda tabloid in remarks published Friday.