The United States Saturday demanded that Syrian President Bashar al-Assad's forces halt their bombardment of Aleppo and help restore a nationwide ceasefire, with Secretary of State John Kerry due to head to Geneva for talks on the conflict.
Terrified residents fled a new wave of air strikes on rebel-held areas of the divided city as key regime backer Russia rejected calls to rein in its ally.
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.
In calls to De Mistura and the lead Syrian opposition negotiator, Kerry expressed "deep concern" about Aleppo, which has suffered some of the worst fighting in a conflict that has killed more than 270,000 people and displaced millions.
"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.
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 besieged city of Aleppo where nearly 250 people have been killed in the last 10 days.
In the calls, Kerry dismissed Russian and regime claim that the Aleppo strikes were targeting the Al-Nusra Front, a jihadist 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.
Aleppo was left out of a new temporary US-Russian brokered truce that appeared to be holding in the regime stronghold of Latakia as well as Damascus and the nearby rebel bastion of Eastern Ghouta.
A new round of UN-backed peace talks is set to start on May 10 in Geneva.
- 'Unbearable situation' -
In Aleppo's rebel-held east, dozens of civilians left the battered Bustan al-Qasr district early Saturday, an AFP correspondent said.
"The situation has become unbearable," Abu Mohammed said as he prepared to flee with his wife and five children. "Everything is paralysed."
Russia said that it would not ask Damascus to halt air raids on Aleppo.
"No, we are not going to put pressure on (Damascus) because one must understand that the situation in Aleppo is part of this fight against the terrorist threat," Deputy Foreign Minister Gennady Gatilov said.
At least 246 civilians have died in shelling, rocket fire and air strikes in both sides of the city since April 22, the Syrian Observatory for Human Rights said.
At least 10 civilians died in rebel-controlled areas on Saturday, according to the civil defence.
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The violence in Aleppo has severely tested the February 27 truce between the regime and non-jihadist rebels intended to pave the way to an end to the five-year conflict.
The few people out on the streets watched the sky anxiously for regime aircraft, running for shelter when one launched a new raid.
The Britain-based Observatory, which relies on a network of sources on the ground, reported 28 air strikes on eastern neighbourhoods.
But in its daily report on Syria, Russia's defence ministry said it had recorded only "three ceasefire violations in the city of Aleppo", blaming them all on the rebels.
The SANA state news agency said shelling of western government-held neighbourhoods killed three civilians, including a child, and blamed Al-Qaeda affiliate Al-Nusra Front and its allies.
A pro-government newspaper said Thursday the army was preparing an offensive to recapture all of Aleppo and the surrounding province.
- Aid for besieged towns -
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.
On other fronts, fighting halted at 1:00 am Saturday (2200 GMT Friday) in a "freeze" set to last for 24 hours in Damascus and Eastern Ghouta, and 72 hours in Latakia.
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.
Anas al-Abdeh, head of the Istanbul-based opposition National Coalition, Saturday accused the regime of "war crimes and crimes against humanity" in Aleppo.
Human Rights Watch also said the air strikes on medical facilities in the city "may amount to war crimes".
Qatar called for an emergency Arab League meeting, and Saudi Arabia condemned the regime strikes.