BEIRUT-- A series of airstrikes hit a hospital and nearby buildings in the rebel-held part of Syria's contested city of Aleppo overnight, killing at least 27 people -- including at least two doctors and three children -- as the U.N. envoy for Syria appealed early Thursday to the U.S. and Russia to help revive the Syrian peace talks and a cease-fire he said "hangs by a thread."

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

A Syrian monitoring group and a first-responders team said hours later that new airstrikes on rebel-held parts of Aleppo had killed another 20 people and brought down at least one residential building. The new violence brought the death toll from the past 24 hours in the deeply divided city to at least 61.

The strikes on the hospital hit shortly before midnight Wednesday, according to opposition activists and rescue workers. They struck a well-known field hospital in the rebel-held district of al-Sukkari in Aleppo. The dead included the last pediatrician remaining in the city's opposition-held areas and a dentist, activists and a British surgeon who knows the hospital told CBS News.

As CBS News correspondent Holly Williams reports, video from the immediate aftermath of the strikes shows dead and wounded children being pulled from the rubble of what was reportedly a hospital specializing in pediatric care.

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The devastating video, adds Williams, is just the latest sign that the peace process that many had hoped weeks ago might finally bring an end to the suffering of the Syrian people, is in tatters.

A high-ranking Syrian military officer denied Thursday that the army had targeted any hospital in Aleppo, telling CBS News the reports were "null and void."

"The heroic Syrian army is fighting terrorism. The army never targeted hospitals or civilians as some media is circulating. This is null and void and aimed at destorting the image and victory of the Syrian army. We will keep up the fight against ISIS, Nusra and other related terrorist groups until the end," said the officer, who spoke to CBS News on condition of anonymity.

The French news agency AFP, meanwhile, cited an anonymous Syrian official as saying the army was in fact preparing to launch a massive offensive to retake the entire city of Aleppo in the coming days.

"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," AFP quoted its source as saying.

Such an operation would sound an almost certain death knell for the flailing peace process, which has had little effect in Aleppo of late. More than 200 people have died in just the last week amid regime bombardments and rebel shelling of government-held portions of the city.

Al-Sukkari, like much of Aleppo, is an area contested by several opposition factions, but is largely held by Western-backed Free Syrian Army rebels. Government forces hold ground further to the south of the sprawling city.

The head of the Britain-based Syrian Observatory for Human Rights said at least 27 were killed, including three children, and that the hospital was completely destroyed.

The Syrian Civil Defense, a volunteer first-responders agency, whose members went to the scene of the attack, 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.

Among those killed were three of the hospital's medical staff, they said.

Dr. David Nott, a U.K. trauma surgeon who worked previously at the al-Quds hospital and who knew the pediatrician killed overnight, told CBS News the strikes leave Aleppo with nobody to provide specialist medical care for the children in the vast rebel-held swathes of Syria's second-largest city.

Nott said there are now only three hospitals left in rebel-controlled Aleppo -- one just 500 yards away from the razed al-Quds facility, and two more that are located underground to offer protection from the relentless bombing at the surface. As of a year ago, Nott said these hospitals were functioning very well. But now, because of a lack of funding, the one that is above ground often has to stop functioning.

International aid agency Doctors Without Borders, known by the French acronym MSF, criticized the strikes against the al-Quds hospital, which the group said it had supported.

"MSF categorically condemns this outrageous targeting of yet another medical facility in Syria," Muskilda Zancada, the group's head of mission in Syria, said in a written statement. "This devastating attack has destroyed a vital hospital in Aleppo, and the main referral centre for paediatric care in the area. Where is the outrage among those with the power and obligation to stop this carnage?"

We condemn the destruction of the Al Quds hospital in #Aleppo, depriving people of essential healthcare. Hospitals are #notatarget, #Syria — MSF International (@MSF) 28 April 2016

The International Committee of the Red Cross also chastised the perpetrators of the airstrike, saying such violence was driving Aleppo closer to an all-out humanitarian crisis.

"The recent attack on the ICRC-supported Quds hospital is unacceptable and sadly this is not the first time the lifesaving medical services have been hit," ICRC Syrian mission chief Marianne Gasser said in a written statement. "We urge all the parties to spare the civilians. Don't attack hospitals, don't use weapons that cause widespread damage. Otherwise, Aleppo will be pushed further to the brink of humanitarian disaster."

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."

It remained unclear whose warplanes or helicopters carried out the airstrikes overnight and on Thursday morning. Russian jets have flown thousands of air raids in support of the Assad regime, which does have a smaller number of aircraft of its own. The U.S. and its international allies have also been conducting airstrikes in the country for months, but all targeting the Islamic State of Iraq and Syria (ISIS), and al Qaeda affiliate the Nusra Front.

Russian Defense Ministry spokesman Maj. Gen. Igor Konashenkov denied bombing any hospitals in Aleppo, saying its aircraft have not flown any missions in the region for several days.

Col. Steve Warren, the spokesman for the U.S.-led campaign against ISIS, said fighter jets from the international coalition have not carried out any airstrikes in Aleppo in the past 24 hours.

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 U.N. envoy for Syria, Staffan de Mistura, briefed the U.N. 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.