The city of Aleppo is at the epicenter of a military escalation that has undermined peace talks in Geneva to end the five-year-old war and UN envoy Staffan de Mistura appealed to the presidents of the United States and Russia to intervene.

Six days of air strikes and rebel shelling in Aleppo, which is split between government forces and rebels, have killed some 200 people in the city, two-thirds of them on the opposition side, the Syrian Observatory for Human Rights says.

"The catastrophic deterioration in Aleppo over the last 24-48 hours" has jeopardized the aid lifeline that delivers supplies to millions of Syrians, said Jan Egeland, chairman of the UN humanitarian task force. "I could not in any way express how high the stakes are for the next hours and days."

The Geneva talks aim to end a war that has killed more than 250,000 people, created the world's worst refugee crisis, allowed for the rise of Islamic State and drawn in regional and major powers but the negotiations have all but failed and a truce to allow them to take place has collapsed.

Winding up the Geneva talks, de Mistura said he aimed to resume them in May, but gave no date.

"Wherever you are, you hear explosions of mortars, shelling and planes flying over," Valter Gros, who heads the International Committee of the Red Cross Aleppo office, said.

"There is no neighborhood of the city that hasn’t been hit. People are living on the edge. Everyone here fears for their lives and nobody knows what is coming next," he said.

A Syrian military source said government planes had not been in areas where air raids were reported. Syria's army denied reports that the Syrian air force targeted the hospital.

The Russian defense ministry, whose air strikes have swung the war in favor of President Bashar al-Assad, could not immediately be reached for comment. Russia has previously denied hitting civilian targets in Syria where it launched air raids late last year to bolster its ally.

The British-based Observatory said 31 people were killed as a result of air strikes on several areas of opposition-held Aleppo on Thursday. In addition, it said at least 27 people were killed in the air strike on the hospital that was struck late on Wednesday. Rescue workers put the toll higher.

In government-held areas, rebel mortar shelling killed at least 14 people, the Observatory and Syria's state news agency SANA reported.

"WHERE IS THE OUTRAGE?"

The bombed al-Quds hospital was supported by international medical charity Medecins Sans Frontieres (MSF), which said it was destroyed after being hit by a direct air strike that killed at least three doctors.

“This devastating attack has destroyed a vital hospital in Aleppo, and the main referral center for pediatric care in the area," said Muskilda Zancada, MSF head of mission, Syria. "Where is the outrage among those with the power and obligation to stop this carnage?”

ICRC spokesman Ewan Watson told Reuters in Geneva: "It is unacceptable, any attack on hospitals is a war crime. But it is up to an investigator and it is for a court to take that decision on whether it is a war crime or not."

Peace talks, which have been deeply divided on the future of Assad, looked to be over last week when the opposition walked out, saying the Syrian government was stalling for time to advance on the ground and calling for implementation of a UN resolution requiring full humanitarian access to besieged areas.

De Mistura voiced deep concern at the truce unraveling in Aleppo and at least three other places, but also said he saw some narrowing of positions between the government and opposition visions of political transition.

"Hence my appeal for a US-Russian urgent initiative at the highest level, because the legacy of both President Obama and President Putin is linked to the success of what has been a unique initiative," de Mistura told a news conference.

They should "be able to revitalize what they have created and which is still alive but barely".

The United States and Russia must convene a ministerial meeting of major and regional powers who compose the International Syria Support Group (ISSG), he said.

Egeland said: "So the appeal of Staffan de Mistura to the United States, to Russia and to the other powers in the ISSG is 'you did it once, you can do it again.'"

FUTURE OF ASSAD CRITICAL

Bashar Ja'afari, who led the government delegation, said on Tuesday the round had been "useful and constructive". But he gave no sign of ceding to the opposition HNC's central demand for a political transition without Assad. The government has said the future of Assad is non-negotiable.

De Mistura, asked whether Assad’s fate was discussed, replied: "We didn’t get into names of people ... but actually how to change the current governance."

The UN envoy said the two sides remained far apart in their vision of a political transition, but shared some "commonalities", including the view "that the transitional governance could include members of the present government and the opposition, independents and others".

Giving a chilling statistic about the backdrop of violence against which the talks played out, de Mistura said that in the past 48 hours there had been an average of one Syrian civilian killed every 25 minutes and one wounded every 13 minutes.

Hossam Abu Ghayth, 29, a documentary film-maker living in the rebel-held area of Kalasa in Aleppo which was bombed on Thursday, said by WhatsApp: "There are still planes (flying) ... They're hitting everything, mosques, markets, residential buildings, field hospitals.

"Dozens of people are under the rubble and the Civil Defence cannot dig out the bodies because of the intensity" of the bombardments.

Tony Ishak, 26, a resident of the government-held area of Suleimaniya in Aleppo and a politics student, said via WhatsApp:

"It's been really bad for around four days now, the situation is worse than bad. Shells are falling like rain everywhere. The hospitals are full."