The Syrian army was preparing an offensive to retake the northern city of Aleppo, where 53 civilians were killed in fighting Thursday in a new blow to a tattered truce.
It was the highest death toll for a single day in a week of fighting in the divided city of Aleppo between rebels and regime forces that has cost more than 200 lives, a monitoring group said.
UN envoy Staffan de Mistura warned that a hard-won February 27 ceasefire was now "barely alive", appealing for urgent action by its co-sponsors Russia and the United States to rescue it.
At the Security Council, UN aid chief Stephen O`Brien echoed de Mistura, urging world powers to revive the truce and put an end to the "massive human suffering" in war-torn Syria.
But pro-government newspaper Al-Watan said the army was poised for a "decisive battle" for Aleppo and the surrounding province, while a source close to the regime said the campaign was imminent.
"Now is the time to launch the battle for the complete liberation of Aleppo," the paper said, adding that it "will not take long to begin, nor to finish".
The regime source told AFP that "the army is preparing a huge operation in the coming days to push the rebels away from the city by encircling it and creating a security zone".
Rebels have controlled eastern districts of Aleppo city since 2012, while western neighbourhoods are held by the regime.
Control of the surrounding province is divided between a myriad of armed groups -- jihadists of Al-Qaeda and the Islamic State group, Kurdish militia and various rebel factions as well as the army.
Further north in the province, rebels including the powerful Islamist Ahrar al-Sham group and Kurdish forces fought fierce battles Wednesday and early Thursday that left 64 fighters dead, a monitor said.The upsurge in violence in and around Aleppo has strained the February truce between the government and non-jihadist rebels and cast a shadow over UN efforts to convene a new round of peace talks next month and channel aid to Syria`s beleaguered population.
Rebel rocket and artillery fire on government-held neighbourhoods on Thursday killed 22 civilians, including two children, the Syrian Observatory for Human Rights said.
Regime air strikes on rebel-held districts killed 31 civilians, including three children, according to the British-based monitor.
An AFP correspondent said every building in sight in the Bustan al-Qasr district had had its windows blown out by the air strikes.
"It is the worst day in Aleppo in five years. The regime did not spare a single neighbourhood," said one resident.
The International Committee of the Red Cross warned that Aleppo city was "on the brink of humanitarian disaster".
"Wherever you are, you hear explosions of mortars, shelling and planes flying over," said Valter Gros, who heads ICRC`s office in Aleppo.
The UN aid chief also raised the alarm, urging world powers to salvage the truce.
"We must all be ashamed this is happening on our watch," said O`Brien.
"You must not squander the opportunity presented by talks in Geneva and by the cessation of hostilities to put an end to the massive human suffering in Syria," he said, appealing to world powers.On Wednesday, government air strikes hit a hospital and nearby block of flats in Aleppo`s rebel-held Sukkari neighbourhood, killing 30 people, the civil defence group known as the White Helmets told AFP.
Among the dead was the last paediatrician still working in rebel areas of the city.
Doctors Without Borders (MSF), which had been supporting the hospital, condemned the strike which it said killed 14 people.
"MSF categorically condemns this outrageous targeting of yet another medical facility in Syria," said Muskilda Zancada, the medical charity`s head of mission in Syria.
"This devastating attack has destroyed a vital hospital in Aleppo, and the main referral centre for paediatric care in the area."
A rescue worker said 10 of the dead were so badly disfigured they could not be identified.
Syria`s main opposition group, the High Negotiations Committee, condemned the strike on the hospital as "heartless".
The Aleppo violence has raised fears for the ceasefire in other areas of Syria and called into question the future of UN-brokered peace talks in Geneva that have now gone into recess.
De Mistura said the truce was "in great danger" and could collapse "at any time".
He said the United States, which supports some rebel groups, and regime ally Russia needed to act, calling on them to organise a high-level Syria meeting before negotiations resume.