At least 26 civilians have died in fresh government air strikes on Syria's contested city of Aleppo, activists said.
Meanwhile the United Nations Security Council convened an emergency meeting on the spiralling violence but failed to take any action because of deep divisions between Russia and the Western powers.
The United States, Britain and France, who called the emergency meeting, heaped blame on Moscow for supporting the Syrian offensive which UN envoy Staffan de Mistura called one of the worst of the five-year war.
When Syria's UN ambassador Bashar Ja'afari was called to speak in the council, the ambassadors of the three Western powers walked out in protest.
They had demanded a halt to the Aleppo offensive and immediate council action and their walkout demonstrated anger and frustration not only at Damascus but at Russia for backing close ally Bashar Assad's military campaign while talking about reviving a cessation of hostilities.
"What Russia is sponsoring and doing is not counter-terrorism, it's barbarism," said US ambassador Samantha Power. "It's apocalyptic what is being done in eastern Aleppo."
As the government offensive entered its fourth day on Sunday, medical workers and local officials reported air strikes on neighbourhoods throughout Aleppo's rebel-held eastern districts.
The British-based Syrian Observatory for Human Rights reported 26 civilians had been killed by 7.30pm and said it expected the toll to rise.
Ibrahim Alhaj of the Syrian Civil Defence search and rescue group gave a higher toll, saying hospitals and rescuers had documented the deaths of 43 people.
The observatory, which relies on a network of contacts inside Syria, said earlier that 213 civilians had been killed by air strikes and shelling on opposition areas in and around Aleppo since the US-Russian brokered ceasefire collapsed last Monday.
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 city's opposition quarters.
"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."
Russia's UN ambassador Vitaly Churkin blamed Syria's rebels for sabotaging the ceasefire agreement by using the lull to shore up their forces and he accused the Western coalition of failing to separate the moderate factions it backs from "terrorist" groups especially the al Qaida-linked Fatah Sham Front.
"In Syria, hundreds of armed groups are being armed," Mr Churkin said. "The territory of the country is being bombed indiscriminately. Bringing a peace is almost an impossible task now."
But he made clear that Moscow had not given up on a cessation of hostilities.
"Of course, I would like this very much to be our aim as well as the renewal of negotiations," he said. But he stressed that Russia first wanted to see "a sincere desire" by the US coalition to separate the moderate opposition from the Fatah Sham Front.
Ms Power said the United States knows "that Russia has consistently said one thing, and done the opposite", but said the US also believed it must do "everything in our power to find a way to halt the violence".
While intra-Syria negotiations remain the aim of the Western powers and Russia, a broad coalition of 33 Syrian rebel factions issued a statement on Sunday saying: "Negotiations under the present conditions are no longer useful and are meaningless."
The opposition groups said they would not accept any Russian mediation, calling Moscow a "partner to the regime in the crimes against our people".
They also called on the government and Russian forces to halt air strikes and lift sieges on opposition areas where the UN estimates 600,000 Syrians are trapped.
On the sidelines of the meeting, UN secretary general Ban Ki-moon urged an end to the offensive saying the bombs "are not busting bunkers, they are demolishing ordinary people looking for any last refuge of safety".
"International law is clear: The systematic use of indiscriminate weapons in densely-populated areas is a war crime," he said.
Before the start of the UN meeting, Foreign Secretary Boris Johnson said Russia should be investigated for war crimes following the attack on the aid convoy that claimed 20 lives.
And France's foreign minister Jean-Marc Ayrault said Russia and Iran would be guilty of war crimes if they did not pressure Assad to stop escalating violence.
Rebels, meanwhile, shelled Masyaf, a government stronghold near the central city of Hama, for the second day in a row on Sunday, according to the Syrian Observatory.
Masyaf is home to a large number of Alawites, members of Assad's sect. The Syrian leader has rallied Syria's minorities behind his government behind fears of the Sunni-dominated rebellion.
On a positive note, a set of four towns, two besieged by government forces and two by rebels, were reached by aid convoys for the first time in nearly six months on Sunday, the International Committee for the Red Cross announced.
The organisation said it had reached 60,000 residents trapped in the towns of Madaya, Zabadani, Foua, and Kafraya.