Syria's Aleppo faced worsening food and medical shortages and warplanes again pounded the city as Russia condemned what it called "unacceptable" Western rhetoric after allegations of war crimes.
A fresh wave of intense air strikes battered Aleppo's opposition-controlled east, said an AFP correspondent in the city facing its worst violence in years.
During an emergency session of the UN Security Council, US ambassador Samantha Power accused Russia of "barbarism", while the British and French envoys went even further.
"War crimes are being committed here in Aleppo," Francois Delattre of France said, while Britain's envoy spoke of bunker-busting bombs and more sophisticated weaponry unleashing a "new hell" on Syrians.
"It is difficult to deny that Russia is partnering with the Syrian regime to carry out war crimes," said Britain's Matthew Rycroft.
The Kremlin hit back.
Spokesman Dmitry Peskov denounced "the overall unacceptable tone and rhetoric of the representatives of the United Kingdom and the United States, which can damage and harm our relations".
Despite the exchange, the violence showed no signs of abating on the ground, with people in Aleppo saying food and vital medical supplies were dwindling to nothing.
The Syrian Observatory for Human Rights monitor said at least 12 people, including three children, were killed in Monday's raids on several rebel-held districts.
- Blood shortage, more amputations -
Including the latest deaths, a total of 140 people, nearly all civilians, have been killed in Syrian and Russian raids on eastern Aleppo since late Thursday.
They include 23 children and 10 women, Observatory director Rami Abdel Rahman said Monday.
It was the fourth day of intense bombardment since a defiant Syrian regime launched a new assault to retake all of Aleppo following the collapse early last week of a short-lived ceasefire brokered by Moscow and Washington.
Since the truce fell apart, a total of 248 people have been killed in Aleppo city and the wider province by Russian and government bombardment, the Observatory said.
A Syrian military source told AFP regime forces had no intention of letting up on rebel-held areas.
"The air force will bomb any terrorist movements, this is an irreversible decision," the source said, reiterating the regime's goal was to "recapture all regions of Syria" outside its control.
A medical source in rebel-held Aleppo said hospitals were struggling to deal with a huge number of casualties.
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"Hospitals that are still in service are under a lot of pressure due to the significant number of wounded in recent days, and the major shortage of blood," the source told AFP.
"Because of this, serious injuries are requiring immediate amputations."
With Aleppo back under siege since regime forces again fully surrounded the city in early September, residents were left reeling from food shortages and skyrocketing prices as well as intensifying violence.
- 'End the nightmare' -
Several charity kitchens in Aleppo's rebel-held east were closed in fear of strikes, while water remained cut after pumping stations were damaged at the weekend.
"We endured through years of bombardments and did not leave Aleppo. But now there is no bread, no drinking water, nothing in the markets. The situation is getting worse every day," said Hassan Yassin, a 40-year-old father of four.
On Monday, dozens of rebels and their families quit the last opposition-held district of central Homs city as part of a deal struck with the regime last year.
A total of 131 fighters and 119 family members were bussed out of Waer, devastated after a three-year regime siege, to rebel-held Dar al-Kubra further north, said a source from Homs governorate.
An estimated 600,000 Syrians live under siege, according to the UN, with most encircled by regime forces though rebels also use the tactic.
The UN's World Food Programme said it delivered food aid on Sunday to civilians in four besieged towns in Syria for the first time since April.
Madaya and Zabadani, near Damascus, are encircled by government forces while Fuaa and Kafraya in the northwest are besieged by hardline rebels.
A convoy of 53 trucks entered Madaya and Zabadani, with another 18 to Fuaa and Kafraya, according to the International Committee for the Red Cross.
At Sunday's Security Council meeting, US envoy Power voiced some of the strongest criticism yet of Russia's support for President Bashar al-Assad's regime.
"What Russia is sponsoring and doing is not counter-terrorism. It is barbarism," she said.
Russia's year-long air war has helped Assad's forces regain ground lost to a wide range of opposition forces.
UN Secretary General Ban Ki-moon has also warned the use of advanced weaponry against civilians could amount to war crimes.
Ban called on world powers to "work harder for an end to the nightmare" in Syria that has killed more than 300,000 people and driven millions from their homes.