At least 23 civilians were killed in renewed government airstrikes on the contested city of Aleppo as the United Nations Security Council convened an emergency meeting on the spiralling violence in Syria.
At the start of that meeting the UN's top envoy to Syria accused the government of unleashing "unprecedented military violence" against civilians in Aleppo.
Staffan de Mistura said Syria's declaration of a military offensive to retake rebel-held eastern Aleppo has led to one of the worst weeks of the five and a half year war with dozens of airstrikes against residential areas and buildings causing scores of civilian deaths.
He said the offensive targeting civilians with sophisticated weapons, including incendiary devices, may amount to war crimes.
Medical workers and local officials reported airstrikes on areas throughout Aleppo's rebel-held eastern districts as an announced government offensive entered its fourth day.
The Britain-based Syrian Observatory for Human Rights reported 23 civilians had been killed by nightfall and said it expects the toll to rise.
Ibrahim Alhaj, of the Syrian Civil Defence search and rescue outfit, said hospitals and rescuers have documented the deaths of 43 people so far.
Hospitals are overwhelmed with casualties and medical workers are expecting many of the wounded to die from a lack of treatment, according to Mohammad Zein Khandaqani, a member of the Medical Council, which oversees medical affairs in the opposition areas.
"I've never seen so many people dying in once place," he said from a hospital in the city. "It's terrifying today. In less than one hour the Russian planes have killed more than 50 people and injured more than 200."
The Observatory, which relies on a network of contacts inside Syria, said earlier in the day that 213 civilians had been killed by airstrikes and shelling on opposition areas in and around Aleppo since a US-Russian brokered ceasefire collapsed on Monday evening.
Mr de Mistura, at the Security Council meeting, warned that if the Syrian government is intent on taking Aleppo, it is going to be "a grinding" street-by-street fight where all the infrastructure in the city will be destroyed, but it will not lead to victory.
"A so-called military solution is impossible including in Aleppo," he stressed.
He urged the US and Russia to go "that extra mile" and save the September 9 cessation of hostilities agreement "at the 11th hour".
Prior to the start of the UN meeting, British Foreign Secretary Boris Johnson said Russia should be investigated for war crimes following an attack on a Syrian aid convoy that claimed 20 lives.
Mr Johnson said that Russia's air force may have deliberately targeted the civilian convoy, though Russia denies involvement and instead suggests Syrian rebels or a US drone were responsible.
Horror in Aleppo. Incessant Russian attacks this weekend kill over 500, & insufficient beds in makeshift hospitals to treat injured. pic.twitter.com/3w1vHvzL3O— Kenan Rahmani كنان (@KenanRahmani) September 25, 2016
France's Foreign Minister Jean-Marc Ayrault said Russia and Iran will be guilty of war crimes if they don't pressure Syrian President Bashar Assad to stop escalating violence.
Mr Ayrault said the emergency Security Council meeting on Sunday was a "moment of truth" for the UN.
The meeting was requested by the United States, Britain, and France, as pro-government forces extend their bombardment of the contested city of Aleppo. They are widely believed to be accompanied by Russian air strikes.
Rebels meanwhile shelled Masyaf, a government stronghold near the central city of Hama, for the second day in a row, according to the Observatory.
Masyaf is home to a large number of Alawites, members of President Bashar Assad's sect. Mr Assad has rallied Syria's minorities behind his government behind fears of the Sunni-dominated rebellion.