At least 20 people have been killed in a Syrian regime air strike on a hospital in the city of Aleppo as the United Nations warned that a delicate ceasefire was crumbling, according to local sources.

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 opposition-held part of the city.

Rescue workers were pulling bodies and survivors from the rubble of the building in the Sukkary neighborhood into the evening and witnesses told Al Jazeera the number of dead was likely to rise.

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

The attack was the latest in an intensification of regime assaults on the city, with at least 100 civilians killed in air strikes, shelling and rocket fire since Friday.

"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 107 civilians, including 20 children and 13 women, between last Friday and Wednesday as violence between regime forces and opposition groups escalated.

On Tuesday, at least 35 people - including eight children - were killed in Aleppo in fighting between regime forces and the opposition, 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 regime 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 regime forces pressed on with an offensive against opposition fighters there.

A ceasefire unravelling

De Mistura on Thursday said that the regime 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 regime and the opposition, independents and others".

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.

PHOTO CAPTION

Civil defense members search for survivors after an airstrike at a field hospital in the opposition held area of al-Sukari district of Aleppo, Syria April 27, 2016 [Reuters].

Al-Jazeera