The United States called Russia's action in Syria "barbarism" while Moscow's UN envoy said ending the war "is almost an impossible task now" as government forces relentlessly bombed the besieged city of Aleppo.

The United Nations Security Council met on Sunday at the request of the United States, Britain and France to discuss the escalation of fighting in Aleppo following the announcement on Thursday of an offensive by the Syrian army to retake the embattled northern city.

"What Russia is sponsoring and doing is not counter-terrorism, it is barbarism," the US ambassador to the UN, Samantha Power, told the 15-member council.

"Instead of pursuing peace, Russia and Assad make war. Instead of helping get life-saving aid to civilians, Russia and Assad are bombing the humanitarian convoys, hospitals, and first responders who are trying desperately to keep people alive," said Power.

How critical is the city of Aleppo in Syria's war?

A September 9 ceasefire deal orchestrated by US and Russia aimed at putting Syria's peace process back on track effectively collapsed on Monday when an aid convoy was bombed north of Aleppo.

"In Syria hundreds of armed groups are being armed, the territory of the country is being bombed indiscriminately, and bringing a peace is almost an impossible task now because of this," Russia's UN Ambassador Vitaly Churkin told the council.

Britain's UN Ambassador Matthew Rycroft said the bid by the US and Russia to bring peace to Syria "is very, very near the end of its life and yes the Security Council needs to be ready to fulfill our responsibilities".

"The regime and Russia have instead plunged to new depths and unleashed a new hell on Aleppo," Rycroft told the council. "Russia is partnering with the Syrian regime to carry out war crimes."

Russia is one of five veto-powers on the UN Security Council, along with the US, France, Britain and China. Russia and China have protected Syrian President Bashar al-Assad's government by blocking several attempts at council action.

"It is time to say who is carrying out those air strikes and who is killing civilians. Russia holds a permanent seat on the UN Security Council, this is a privilege and it is a responsibility. Yet in Syria and in Aleppo, Russia is abusing this historic privilege," Power said.

As Syria's UN Ambassador Bashar Jaafari began addressing the council, Power, Rycroft and French UN Ambassador Francois Delattre walked out of the chamber.

Jets pound Aleppo

The emergency UN meeting came as Syrian government and Russian air strikes continued to pummel rebel-held areas of Aleppo in some of the heaviest bombardments since the country's conflict began in 2011. 

More than 60 people were killed on Sunday in air strikes on rebel-held neighbourhoods of the divided city, according to members of the Syrian Civil Defence, a volunteer search-and-rescue group also known as the White Helmets.

It lost several vehicles in Sunday's bombardment and was struggling to reach many of those caught up in bombings across the city and its outskirts. 

"We don't have enough vehicles to cover the whole city. [Assad's forces] bombed us directly and devastated most of our headquarters ... but we'll continue with our duty until the end," Ahmad, an Aleppo-based volunteer with the rescue group, told Al Jazeera.

READ MORE: 'Ferocious' air strikes pummel Aleppo as ground gained

At least 231 civilians have been killed in violence in Aleppo and its outskirts since a truce collapsed last week amid an intense air campaign by the government and its ally Russia, according to the UK-based Syrian Observatory for Human Rights. 

'Tiny window of opportunity'

At the Security Council meeting on Sunday, UN Syria envoy Staffan de Mistura urged members to come up with a way to enforce the failed cessation of hostilities.

The US and Russia still have a "tiny window of opportunity" to salvage the now broken-down truce, said de Mistura. But he noted mutual trust to solve the five-year crisis was "seriously broken".

Aleppo residents plead for help as Syria fighting rages

"I want to believe, because I am still a naive UN official, that they really meant what they said ... and they really wanted it to work," de Mistura added.

"I'm asking, indeed urging, both of them to go that extra mile to see if they can save their agreement ... and do so at the 11th hour."

Noting attacks of "unprecedented" frequency and intensity in eastern Aleppo, de Mistura called on the council to ensure a cessation of hostilities against civilians, the establishment of weekly 48-hour humanitarian pauses in fighting, and medical evacuations.

The surge in violence has worsened the situation inside eastern Aleppo, which has been besieged by government forces since July 17.

The siege has resulted in acute shortage of food and medical supplies as well as fuel, according to the Syrian Observatory, which relies on a network of activists inside Syria.

A UN official on Saturday noted nearly two million people had been cut off from running water. 

Zouhir al-Shimale in Aleppo contributed to this report 

Source: Al Jazeera News And Agencies