Russia said Sunday it was working to freeze fighting in Syria's Aleppo as US Secretary of State John Kerry was due in Geneva in a bid to restore a nationwide ceasefire.

More than a week of fighting in Syria's second city has killed several hundred civilians and left a UN-backed peace process hanging by a thread.

Concern has been growing that the fighting in Aleppo will lead to the complete collapse of a landmark ceasefire between President Bashar al-Assad's regime and non-jihadist rebels that was brokered by Moscow and Washington.

On Saturday Moscow said it would not urge Assad's forces to halt air raids on the war-ravaged city as they were targeting jihadist groups not covered by the ceasefire, which took effect in late February.

But on Sunday, after Washington called on Russia to push its Syrian ally to end the strikes, the head of Moscow's coordination centre in Syria said talks on a freeze had begun.

"Currently active negotiations are underway to establish a 'regime of silence' in Aleppo province," Lieutenant General Sergei Kuralenko was quoted as saying by Russian news agencies.

He said that a freeze in fighting in Eastern Ghouta, a suburb of Damascus, had been extended by another 24 hours to the end of Sunday and that another freeze was holding in northern Latakia province.

"We are calling on all sides interested in establishing peace in Syria to support the Russian-American initiative and not to allow a regime of silence to be disrupted," Kuralenko said, speaking from Russia's Hmeimim air base in Syria.

Kerry was due to arrive in Geneva later on Sunday for talks with UN envoy Staffan de Mistura and the Saudi and Jordanian foreign ministers on reviving the ceasefire.

In calls to De Mistura and the lead Syrian opposition negotiator on Saturday, Kerry expressed "deep concern" about Aleppo, which has suffered some of the worst fighting in a war that has killed more than 270,000 people and displaced millions.

Kerry made clear that ending the violence in Aleppo and returning ultimately to a durable, nationwide ceasefire was a top priority, spokesman John Kirby said.

At least 253 civilians -- including 49 children -- have died in shelling, rocket fire and air strikes in both sides of the divided city since April 22, according to the Syrian Observatory for Human Rights monitoring group.

Early on Sunday the provincial capital's rebel-held east was eerily quiet after days of bombing, an AFP correspondent said, with the last air strike there at around 1:00 am (2200 GMT Saturday) local time.

There were no reports of fighting in the city's regime-controlled west, the Observatory said.

Air strikes did hit several rebel areas on the city's northwestern outskirts, after rebel shelling of the city's west and regime bombing north of the city in the night, the Observatory said.

The Syrian army said Sunday it was extending the freeze in fighting in Damascus and Eastern Ghouta to 1:00 am Monday (2200 GMT Sunday).

The same "freeze" was set to hold until 1:00 am Tuesday in Latakia, a regime stronghold.

The fighting has dampened hopes that the ceasefire could finally lay the groundwork for an end to Syria's five-year conflict.

Peace talks last month in Geneva failed to make any headway, though De Mistura has said he hopes they can resume "during the course of May".

Concern has also been growing for the battered residents of Aleppo, once Syria's main economic hub.

On Saturday many terrified residents fled a new wave of air strikes on the city's east saying the violence had become unbearable.

Online on Sunday a hashtag demanding an end to the violence -- #AleppoIsBurning -- continued to spread on social media, with protests planned this week around the world.

The escalating violence in the city -- which has been divided since 2012 -- has also hit medical centres.

Four medical facilities were hit Friday on both sides of the front line, the International Committee of the Red Cross said.

And a raid on Wednesday hit a hospital supported by Doctors Without Borders and the Red Cross as well as nearby housing, killing 30 people and 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.

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