Fresh air strikes pummelled the Syrian city of Aleppo on Monday as US Secretary of State John Kerry made a desperate bid to salvage a two-month ceasefire in the war-torn country.
Arriving in Geneva late on Sunday, Kerry was to hold talks with UN peace envoy Staffan de Mistura and the Saudi foreign minister, but the absence of Russia cast a pall over the proceedings.
Washington and Moscow are the joint sponsors of the Syrian peace process, and de Mistura has made it clear that he sees little hope of progress without their agreement.
But Russia, while agreeing in theory to support a ceasefire, has done little to rein in Syrian President Bashar al-Assad's forces around Aleppo, which were in action again on Monday.
More than a week of fighting in and around Syria's second city has killed hundreds of civilians and fresh air strikes hit rebel-held eastern Aleppo in the early hours.
Several neighbourhoods, including the heavily-populated Bustan al-Qasr district, were hit, according to AFP's correspondent in the northern Syrian city.
"What is happening in Aleppo is an outrage. It's a violation of all humanitarian laws. It's a crime," said Saudi Foreign Minister Adel al-Jubeir as he met Kerry.
"It's a violation of all the understandings that were reached," he continued, accusing Assad and the Russians of violating international agreements to back peace.
Kerry was more measured in his response, explaining that Washington will press moderate rebels to separate themselves from the Al-Nusra Front's militants in Aleppo.
Russia and Assad's regime have used the presence of Al-Nusra, which was not party to a February 27 ceasefire deal, as an excuse to press their offensive.
"This is what we're discussing, among other things. There are a number of different ways to approach it," Kerry said, before the envoys went behind closed doors.
"There were a lot of conversations taking place yesterday, the day before, today, and we're getting closer to a place of understanding. But we have some work to do."
Arriving late on Sunday, the top US diplomat had said: "We are talking directly to the Russians, even now.
"The hope is we can make some progress," he said ahead of a meeting with Jordanian Foreign Minister Nasser Judeh.
There is growing concern that the fighting will lead to the complete collapse of the landmark ceasefire agreed between Assad's regime and non-militant rebels.
On Saturday, Russia said it would not push Assad's forces to halt air raids on the war-ravaged city as they were targeting militant groups not covered by the truce.
Washington has rejected this argument and on Sunday the head of Moscow's coordination centre in Syria said talks on a broader freeze had begun.
At least 253 civilians -- including 49 children -- have been killed on both sides of the divided city since April 22, the Syrian Observatory for Human Rights monitor says.
The fighting has dampened hopes that the ceasefire could finally lay the groundwork for an end to Syria's five-year conflict.
Last month's peace talks in Geneva failed to make any headway, though De Mistura has said he hopes they can resume "during the course of May".
Concern for Aleppo, Syria's former economic hub, has been growing.
A hashtag demanding an end to the violence -- #AleppoIsBurning -- has spread on social media, with protests planned around the world this week.
In neighbouring Lebanon, dozens of demonstrators protested, some wearing white helmets marked "Civil Defence" in a nod to Aleppo's rescue workers.
Lebanon currently hosts more than a million Syrian refugees.
The escalating violence in Aleppo has also hit medical centres, with the International Committee of the Red Cross saying four were struck on Friday alone.
Two days earlier, 30 people were killed in when an air strike hit a hospital supported by Doctors Without Borders (MSF) and the Red Cross, sparking an international outcry.
Syria's conflict erupted in 2011 after the brutal repression of anti-government protests and has since escalated into a complex, multi-faceted war.