At least 27 people were killed in a wave of airstrikes on a hospital supported by Doctors Without Borders and nearby buildings in a rebel-held area of Aleppo.

Six hospital staff and three children were among the casualties. The strikes, shortly before midnight Wednesday, hit the well-known al-Quds field hospital in the rebel-held district of Sukkari in Aleppo, according to opposition activists and rescue workers.

On Thursday, at least 20 people were killed by fresh air strikes on rebel-held areas of Aleppo, including three children.

The death toll was expected to rise due to the number of serious injuries, the Syrian Observatory for Human Rights said.

Meanwhile shellfire by insurgents on government-held parts of the northern Syrian city killed at least 14 people including two children, the British-based monitoring group said.

Dozens more were wounded in the bombardments, it said.

The chief Syrian opposition negotiator Mohammed Alloush blamed the government of President Bashar Assad for the deadly airstrikes on the hospital. He told The Associated Press that the latest violence by government forces shows "the environment is not conducive to any political action."

The Syrian military later denied that the air force had targeted the hospital. Russia also denied bombing the site, saying that Russian jets had flown no sorties in the area for several days.

A U.S. spokesperson said in the evening that the bombing bore the hallmarks of previous attacks on medical facilities by the Syrian regime.

Early on Thursday, the UN envoy for Syria appealed to the U.S. and Russia to help revive the peace talks and a cease-fire, which he said "hangs by a thread."

The Civil Defense, a volunteer first-responders agency whose members went to the scene of the attack, put the death toll at 30 and said the dead included six hospital staff. Among those slain was one of the last pediatricians remaining in opposition-held areas of the contested city and a dentist.

The agency, also known as the White Helmets, said the al-Quds hospital and adjacent buildings were struck in four consecutive airstrikes. It said there were still victims buried under the rubble and that the rescue work continued.

The Britain-based Syrian Observatory for Human Rights said at least 27 were killed, including three children.

Doctors Without Borders said in a series of tweets also emailed to the AP that at least 14 patients and staff were among those killed, with the toll expected to rise.

"Destroyed MSF-supported hospital in Aleppo was well known locally and hit by direct airstrike on Wednesday," it said, using a French acronym for the medical charity.

A video posted online by the White Helmets showed a number of lifeless bodies, including those of children, being pulled out from a building and loaded into ambulances amid screaming and wailing. It also showed distraught rescue workers trying to keep onlookers away from the scene, apparently fearing more airstrikes.

Alloush, who was one of the leading negotiators of the opposition in the Geneva talks, described the airstrikes as one of the latest "war crimes" of Assad's government.

"Whoever carries out these massacres needs a war tribunal and a court of justice to be tried for his crimes. He does not need a negotiating table," Alloush told the AP in a telephone interview. "Now, the environment is not conducive for any political action."

The February 27 cease-fire has been fraying in the past weeks as casualty figures from violence mount, particularly in Aleppo and across northern Syria. Airstrikes earlier this week also targeted a training center for the Syrian Civil Defense, leaving five of its team dead in rural Aleppo.

Since April 19, nearly 200 people have died, including at least 44 in an airstrike on a market place in rebel-held area in northern Idlib province, as well as dozens of civilians in government-held areas from rebel shelling.

The UN envoy for Syria, Staffan de Mistura, briefed the UN Security Council via videoconference about the largely stalled indirect talks between the Western- and Saudi-backed opposition and envoys from Assad's government, which has the backing of Moscow.

He said that after 60 days, the cessation of hostilities agreed to by both sides "hangs by a thread."

"I really fear that the erosion of the cessation is unraveling the fragile consensus around a political solution, carefully built over the last year," de Mistura said in his council briefing obtained by The Associated Press. "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."

The talks foundered last week after the main opposition group, called the High Negotiating Committee, suspended its formal participation in the indirect talks with Assad's envoys to protest alleged government cease-fire violations, a drop in humanitarian aid deliveries and no progress in winning the release of detainees in Syria.