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

The United Nations Security Council met on Sunday at the request of the US, Britain and France to discuss the escalation of fighting in Aleppo following an announcement on Thursday of a fresh offensive by the Syrian regime army to retake the northern city, once the country's most populous before the war.

"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," Power said.

A September 9 ceasefire deal brokered by the US and Russia effectively collapsed on Monday when an aid convoy was bombed north of Aleppo.

Britain's UN Ambassador Matthew Rycroft said the US-Russia track was "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 five countries on the UN Security Council with veto powers, along with the US, France, Britain and China. Russia and China have protected Syrian President Bashar al-Assad's regime 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 regime and Russian air raids continued to pummel opposition-held areas of Aleppo in some of the heaviest bombardments since the war began in 2011.

More than 60 people were killed on Sunday in attacks on opposition-held neighborhoods of the divided city, according to members of the Syrian Civil Defense, a volunteer search-and-rescue group also known as the White Helmets.

The group, which operates only in opposition-held parts of the country, 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.

At least 231 civilians have been killed in violence in Aleppo and its outskirts since a truce collapsed last week in an intense air bombardment by the regime 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".

"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 regime 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.

PHOTO CAPTION

Men inspect the damage after an airstrike on the opposition held al-Qaterji neighborhood of Aleppo, Syria September 25, 2016.

REUTERS