BEIRUT — At least 26 civilians were killed in fresh government airstrikes on the contested city of Aleppo, Syrian activists said Sunday, as the United Nations Security Council convened an emergency meeting on the spiraling violence in Syria 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 U.N. envoy Staffan de Mistura called one of the worst of the 5 1/2-year war.

When Syria’s U.N. 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 U.S. 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 airstrikes on neighborhoods throughout Aleppo’s rebel-held eastern districts.

The Britain-based Syrian Observatory for Human Rights reported 26 civilians had been killed by 7:30 p.m. and said it expects the toll to rise. Ibrahim Alhaj of the Syrian Civil Defense search and rescue outfit gave a higher toll, saying hospitals and rescuers had documented the deaths of 43 people so far on Sunday.

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 the U.S.-Russian brokered cease-fire collapsed Monday evening.

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.”

Conflicting casualty estimates are common in the aftermath of clashes and attacks in Syria.

At the start of the Security Council meeting, U.N. envoy de Mistura said the offensive against eastern Aleppo, where up to 275,000 people “have been under a form of de facto siege for almost 20 days,” followed the U.S.-led coalition’s bombing of Syrian troops, which Washington called a tragic mistake, and a deadly attack on a U.N. convoy carrying desperately needed humanitarian aid.

“But no incident, irrespective of whether it can be attributed or not, does justify what is going on in front of our own eyes: the unraveling of the cessation of hostilities and the simultaneous unleashing of unprecedented military violence affecting innocent civilians as well,” he said.

He urged the United States and Russia “to go that extra mile to see if they can save their agreement of Sept. 9 and do so at the eleventh hour.”

He also urged the Security Council to press for a cessation of hostilities, weekly 48-hour pauses to deliver aid, and medical evacuations for several urgent cases in eastern Aleppo.

De Mistura said Syrians and the international community are swiftly losing any remaining hope with Washington and Moscow “unless we salvage what was agreed on.”

“All we can expect from Aleppo if the Syrian government is intent on retaking it is … a slow, grinding, street-by-street fight, over the course of months, if not years, whereby the ancient city will be almost completely destroyed,” he warned.