The UN Security Council met for urgent talks on Sunday as Syrian and Russian warplanes pounded rebel-held east Aleppo in the worst surge of bombing to hit the devastated city in years.
Britain, France and the United States called the emergency meeting to turn up pressure on Russia and press demands that it rein in its ally Syria to halt the intense bombing campaign on Aleppo.
"War crimes are being committed here in Aleppo," French Ambassador Francois Delattre told reporters, adding: "They must not be unpunished and impunity is simply not an option in Syria."
"Just when we thought things couldn't get any worse in Syria, they have," British Ambassador Matthew Rycroft said.
"The incendiary munitions that are dropping on Aleppo are indiscriminate and a clear breach of international law -- the barrel-busting bombs that are falling from the skies likewise," he said.
Residents and a monitor reported heavy air raids overnight and early Sunday on the besieged east of the city, which Syria's army has pledged to retake.
US Ambassador Samantha Power said more than 150 air strikes had hit the city over the past 72 hours. She accused Russia and Syria of launching an "all-out offensive" to re-take Aleppo.
At least 115 people, mostly civilians, have been killed in Syrian and Russian bombardment of eastern Aleppo since the army on Thursday announced an operation to take it, the Syrian Observatory for Human Rights said.
The monitor said at least 19 children were among those killed in the assault, which has included missile strikes, barrel bomb attacks and artillery fire.
Residents said cluster bombs rained down on Saturday night on eastern parts of the city, where an estimated 250,000 people are living under a government siege.
"All night long they were dropping cluster bombs. I couldn't sleep until four in the morning," said 62-year-old Ahmed Hajar, who was out looking for bread in the Al-Kalasseh neighborhood.
"Today the streets of my neighborhood are full of unexploded cluster bombs. One person was killed when he disturbed one and it exploded," he added.
"It tore him apart... it was an awful scene."
In the nearby neighborhood of Bab al-Nayrab, 30-year-old Imad Habush was baking bread in a small wood-burning oven outside his house.
"None of the bakeries are open any more because of the bombing and the shortages of fuel and flour, so people have started making their own bread," he said.
"I don't know why the regime is bombing us in this barbaric way. We're civilians here. We're not carrying weapons, and we're besieged. We have no way to escape."
Air strikes were continuing on the eastern neighborhoods on Sunday, the Observatory said, with at least 14 civilians, including two children, killed since dawn.
Once Syria's economic powerhouse, Aleppo has been ravaged by war and roughly divided since mid-2012 between government control in the west and rebel control in the east.
The eastern portion of the city has been under near-continuous siege since mid-July, causing food and fuel shortages.
A truce deal hammered out by Russia and the United States this month was meant to allow aid to be trucked into the east of the city.
But the ceasefire fell apart before any assistance was brought in. And the army and its Russian ally have since pounded the eastern neighborhoods with a force that residents say is virtually unprecedented.
More than 300,000 people have been killed and over half the country displaced since the war began in March 2011 with anti-government protests.
Successive attempts to reach a political solution have failed, and the latest bid by Moscow and Washington has virtually collapsed, despite ongoing talks to save it.
UN Secretary-General Ban Ki-moon said Saturday he was "appalled by the chilling military escalation" in Aleppo and warned that the use of advanced weaponry could amount to war crimes.
The United States and its European allies said Saturday it was up to Moscow to save the truce.
"The burden is on Russia to prove it is willing and able to take extraordinary steps to salvage diplomatic efforts," read a joint statement from Britain, France, Germany, Italy, the United States and European Union.
"Patience with Russia's continued inability or unwillingness to adhere to its commitments is not unlimited," the statement added.
But Russia has blamed Washington for the ceasefire's failure, saying it did not uphold its commitment to ensure moderate rebels distanced themselves from jihadist groups like former Al-Qaeda affiliate Al-Nusra Front.
Syria's foreign minister said Saturday that his government was confident of "victory" with support from "true friends" including Russia, Iran, and Lebanon's Shiite militant group Hezbollah.
"Our belief in victory is even greater now that the Syrian Arab army is making great strides in its war against terrorism," Walid Muallem told the UN General Assembly.
But on the ground, the army was pushed back from the strategic Handarat camp north of Aleppo city that they captured on Saturday, the Observatory said.