02/05/2016: An earlier version described the hospital attack as being carried out by the Syrian government. This is now updated with the Syrian government denying responsibility.

At least 20 people have been killed in an air strike on a hospital in the city of Aleppo as the United Nations warned that a delicate ceasefire was crumbling.

The attack on the Al Quds hospital on Wednesday killed several medical staff and other civilians, including Dr Wasem Maaz, one of the last remaining pediatricians in the rebel-held part of the city.

At least 14 of those killed were staff and patients for Doctors Without Borders (MSF), the medical aid group, which supported the hospital, said.

Rescue workers were pulling bodies and survivors from the rubble of the building in the Sukkary neighbourhood the following morning on Thursday and witnesses told Al Jazeera the number of dead was likely to rise.

Pictures showed rows of bloodied and charred bodies covered in plastic sheets. The images were too graphic to be published.

A US State Department official said there were indications the hospital bombing was conducted solely by the Syrian government.

However, a Syrian military source told Reuters news agency that 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.

Speaking to Al Jazeera from Damascus, Pawel Krzysiek, a spokesman for the Red Cross, said: "There is no neighbourhood [in Aleppo] that has not been touched by the recent fighting."

"This should not happen," he said, adding that the number of medical facilities left in the city had already been not sufficient to handle the more than two million people in the city.

"The eastern part of the city is the part of Aleppo that has been really hit hard by the fighting," Krzysiek said.

"However, on the western side, you feel the mortars [and] there are thousands of people who fled [other parts of Syria] and are living in the proximity."

The attack was the latest in an escalation of government assaults on the city, with at least 145 civilians killed in air strikes, shelling and rocket fire since Friday.

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"In the last 48 hours, we have had an average of one Syrian killed every 25 minutes, one Syrian wounded every 13 minutes," the UN special envoy to Syria, Staffan de Mistura, said, warning that peace talks were almost out of steam.

In Aleppo alone, the UK-based Syrian Observatory for Human Rights said it had documented the deaths of at least 139 civilians, including 23 children and 15 women, between last Friday and Wednesday as violence between government forces and opposition groups intensfied.

On Tuesday, at least 35 people - including eight children - were killed in Aleppo in fighting between government forces and rebels, a monitoring group said.

The bloc representing the opposition at peace talks in Geneva, the High Negotiations Committee (HNC), on Wednesday threatened to boycott the next round of talks unless the government stopped its bombing campaign.

The HNC left formal negotiations last week, saying that it needed a "pause", after at least 40,000 people fled fighting near Aleppo when government forces pressed on with an offensive against rebel fighters there.

A ceasefire unravelling

De Mistura on Thursday said that the government and the main opposition group remained far apart in their competing visions of a political transition, despite some common ground.

In a seven-page document issued at the end of a two-week round of talks, he said the two sides shared a view "that the transitional governance could include members of the present government and the opposition, independents and others".

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But the potential presence of Assad in such an arrangement has been a sticking point at the talks.

De Mistura called on Russia and the United States to intervene and revive the negotiations.

"I really fear that the erosion of the cessation is unravelling the fragile consensus around a political solution, carefully built over the last year," de Mistura said in a briefing to the UN Security Council, which was obtained by the Associated Press news agency.

"Now I see parties reverting to the language of a military solution or military option. We must ensure that they do not see that as a solution or an option."

With additional reporting by Zouhir Al Shimale in Aleppo.