At least 23 civilians have been killed in renewed regime airstrikes on the contested city of Aleppo, Syrian activists said Sunday, as France and Britain's Foreign Ministers raised the prospect of investigating Russia for war crimes, ahead of an emergency U.N. Security Council meeting about the spiraling violence in Syria.

Medical workers and local officials reported airstrikes on neighborhoods throughout Aleppo's opposition-held eastern districts as an announced regime offensive entered its fourth day.

The Britain-based Syrian Observatory for Human Rights reported 23 civilians had been killed by 6 p.m. and said it expects the toll to rise.

Ibrahim Alhaj of the Syrian Civil Defense 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 have been killed by airstrikes and shelling on opposition areas in and around Aleppo since a U.S.-Russian brokered cease-fire collapsed Monday evening.

At the U.N., 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, Monday.

Johnson said that Russia's air force may have deliberately targeted the civilian convoy on Sept. 19. Russia denies involvement and instead suggests the Syrian opposition or a U.S. drone were responsible.

France's Foreign Minister Jean-Marc Ayrault said Russia and Iran will be guilty of war crimes if they don't pressure Bashar Assad to stop escalating violence.

Ayrault said an emergency United Nations Security Council meeting Sunday is a "moment of truth" for the U.N.

The meeting was requested by the United States, Britain, and France, as pro-regime forces extend their bombardment of the contested city of Aleppo. They are widely believed to be accompanied by Russian air strikes.

Assad has rallied Syria's minorities behind his regime behind fears of a Sunni-dominated rebellion.

The U.S., Britain, and France are aligned on the Security Council against Russia and China, which back Assad in the country's protracted war, now in its sixth year.

But a broad coalition of Syrian rebels denounced international negotiations for peace as "meaningless," earlier Sunday.

The statement released jointly by 33 factions called on the government and Russian forces to halt airstrikes and lift sieges on opposition areas. The U.N. estimates 600,000 Syrians are trapped in various sieges enforced by the regime and Daesht across the country.

"Negotiations under the present conditions are no longer useful and are meaningless," the statement said.

The factions said they would not accept to have Russia mediate any negotiations, calling it a "partner to the regime in the crimes against our people."

The statement was signed by some of the largest factions from across Syria but did not include the powerful, ultraconservative Ahrar al-Sham nor the al-Qaida-linked Fatah Sham Front.

Efforts to revive the truce have floundered. An airstrike destroyed a U.N.-backed humanitarian convoy Monday inside opposition territory shortly after the Syrian regime military announced the agreement had expired. The U.N. says the attack could amount to a war crime if proven deliberate, though it has not assigned responsibility yet. The U.S. says it believes Russian jets were behind the strikes.

The opposition Sunday retook an area in Aleppo that fell to regime forces the day before, extending a punishing stalemate in the contested northern city.

The Observatory says the opposition seized Handarat, a largely uninhabited former Palestinian refugee camp, early Sunday — a day after it was lost to government forces. The camp has changed hands multiple times and is largely devastated and abandoned.

The area is near Castello Road, a vital supply route to the city's besieged opposition-held areas. Regime forces seized the Castello Road earlier this year, besieging opposition-held districts where some 250,000 people reside.