Hashtag demanding end to violence - #AleppoIsBurning - has spread on social media, with protests planned this week around world
US Secretary of State John Kerry arrived Sunday in Geneva in a bid to restore a nationwide ceasefire in Syria, as Russia said it was working to freeze fighting in Aleppo.
More than a week of fighting in Syria's second city has killed hundreds of civilians and left a UN-backed peace process hanging by a thread.
Concern has been growing that the fighting will lead to the complete collapse of a landmark ceasefire between President Bashar al-Assad's government and rebels that was brokered by Moscow and Washington.
On Saturday, Russia said it would not urge Assad's forces to halt air raids on the war-ravaged city as they were targeting militant groups not covered by the truce, 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.
More than 250 civilians dead
"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 from Russia's Hmeimim air base in Syria.
Kerry arrived Sunday night in Geneva, where he will hold talks with UN Syria envoy Staffan de Mistura and the Saudi and Jordanian foreign ministers on reviving the ceasefire.
Before leaving Riyadh for Geneva, Saudi Foreign Minister Adel al-Jubeir accused the Assad government of committing "war crimes" in Aleppo.
Ahead of his Switzerland trip, Kerry expressed his "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.
At least 253 civilians - including 49 children - have died in shelling, rocket fire and air strikes in both sides of the divided city since 22 April, the Syrian Observatory for Human Rights monitor says.
Early on Sunday, the provincial capital's rebel-held east was eerily quiet, an AFP correspondent said, with the last air strike there at around 1am (2200 GMT Saturday) local time.
There were no reports of fighting in the government-controlled west, the Observatory said.
Air strikes did hit several rebel areas on Aleppo's northwestern outskirts, after rebel shelling of the west and government bombing north of the city in the night, the Observatory said.
Rockets were fired Sunday from rebel-held areas into the west without causing casualties, it said.
The government dropped barrel bombs on Castello road, the only route in and out of rebel-held parts of Aleppo, used to move supplies and provide civilians with an escape route, the Observatory said.
Elsewhere, 16 members of the government security forces were killed in attacks by the Islamic State (IS) group near the Shaer gas field in central Homs province and in the nearby Huweiss region, the Observatory said. Seven militants were also killed.
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 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 this week around the world.
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 struck medical centres, with the International Committee of the Red Cross reporting four hit Friday on both sides.
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.
On Saturday, many terrified residents fled a new wave of air strikes on the city's east, saying the violence had become unbearable.
Syria's conflict erupted in 2011 after the brutal repression of anti-government protests and has since escalated into a complex, multi-faceted war.