'Deep concern' about Syria's divided city of Aleppo, where residents have fled new wave of government air strikes on rebel-held areas
The US on Saturday demanded that Syrian President Bashar al-Assad's forces halt their bombardment of the city of Aleppo and help restore a nationwide ceasefire.
In calls to the UN peace envoy for Syria and to the lead opposition negotiator, US Secretary of State John Kerry said the priority was "a durable, nationwide cessation".
A truce was called in February between Assad's forces and a coalition of rebels but has since begun to break down, particularly in the divided and besieged city of Aleppo.
In the past few days, Russia and the US agreed to pressure the parties to hold their fire in the Latakia and Eastern Ghouta regions, but Aleppo was left out of the deal.
Fierce bombing has continued in the city, with heavy civilian casualties, and Russia has made it clear it has no intention of reining in its ally, Assad's forces.
With the peace process hanging by a thread, Kerry was to fly to Geneva on Sunday for talks with UN envoy Staffan de Mistura and the Saudi and Jordanian foreign ministers.
First, he used his calls to emphasise that Washington has not conceded the idea Aleppo can be excluded from the ceasefire nor that civilians can be targeted.
In calls to De Mistura and the general coordinator of the Syrian opposition High Negotiations Committee, Riad Hijab, Kerry expressed "deep concern" about Aleppo.
"The secretary made clear that ending the violence in Aleppo and returning ultimately to a durable, nationwide cessation is a top priority," spokesman John Kirby said.
In the calls, Kerry dismissed the Russian and government claim that the Aleppo strikes are targeting the al-Nusra Front, a militant force that is not party to the ceasefire.
"The secretary made clear that we urged Russia to take steps to stop regime violations, especially its indiscriminate aerial attacks in Aleppo," Kirby said.
"The Assad regime continues to escalate the conflict by predominantly targeting innocent civilians and parties to the cessation of hostilities - not Nusra, as the regime falsely claims.
"Such attacks are direct violations of the cessation and must stop immediately," he declared.
"The secretary underscored that the initial efforts to reaffirm the cessation of hostilities in Latakia and Eastern Ghouta are not limited to these two areas and that efforts to renew the cessation must and do include Aleppo."
Shelling, rocket fire, airstrikes
Once Syria's economic hub, Aleppo and its surrounding countryside have suffered some of the worst fighting in a conflict that has killed more than 270,000 people and displaced millions.
At least 246 civilians have died in shelling, rocket fire and airstrikes in both sides of the city since 22 April, the Syrian Observatory for Human Rights monitor said.
Anas al-Abdeh, head of the Istanbul-based opposition National Coalition, Saturday accused the Syrian government of "war crimes and crimes against humanity" in Aleppo.
Men carry body on stretcher amid rubble of destroyed buildings after airstrike on rebel-held neighbourhood in Aleppo (AFP)
Hospitals have also been bombed in nine days of escalating violence in Aleppo.
Four medical facilities were hit Friday on both sides of the front line, the International Committee of the Red Cross said.
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.
Human Rights Watch also said the air strikes on medical facilities in the city "may amount to war crimes".
Arab League meeting
Qatar called for an emergency Arab League meeting, and Saudi Arabia condemned the the Assad government strikes.
A pro-government newspaper said Thursday that the army was preparing an offensive to recapture all of Aleppo and the surrounding province.
Some families have fled to safer districts, while others left by the dangerous Castello road, the only route out of near-besieged east Aleppo.
Humanitarian convoys carrying food and medicine meanwhile entered the besieged rebel-held towns of Madaya and Zabadani, northwest of Damascus, on Saturday, the Red Cross said.
At the same time, trucks entered the besieged government-held towns of Fuaa and Kafraya, southwest of Aleppo.
Madaya became infamous in late 2015 after dozens died of starvation there.
The violence in Aleppo has severely tested the 27 February truce between the government and rebels intended to pave the way to an end to the five-year conflict.