Médecins Sans Frontières has condemned a series of airstrike that have killed at least 20 people in the Syrian city of Aleppo overnight.

The dead included two doctors and three children.

In a tweet, MSF Syria said it was "outraged at the destruction of Al Quds hospital in Aleppo".

#MSF is outraged at the destruction of Al Quds hospital in #Aleppo. Hospitals are #notatarget pic.twitter.com/TQ1hGhykSp — أطباء بلا حدود سوريا (@MSF_Syria) April 28, 2016

MSF Syria said the hospital was the main referral centre for paediatric patients and included an emergency room, and operating theatre and an intensive care unit that had all been destroyed.

.@MSF supported hospital in #Aleppo was the main referral centre for paediatrics & had 8 doctors & 28 nurses. Hospitals are #notatarget — أطباء بلا حدود سوريا (@MSF_Syria) April 28, 2016

The destroyed MSF-supported hospital in #Aleppo had an ER, an OPD, intensive care unit & an operating theatre. All now destroyed #notatarget — أطباء بلا حدود سوريا (@MSF_Syria) April 28, 2016

It comes as the UN envoy for Syria appealed early Thursday on the US 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".

The strikes hit shortly before midnight on 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 one of the few pediatricians remaining in the city's opposition-held areas and a dentist, activists said.

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The head of the Britain-based Syrian Observatory for Human Rights said at least 20 were killed, including three children, and that the hospital was completely destroyed.

The Syrian Civil Defence, 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.

The agency, also known as the White Helmets, gave a slightly higher toll, saying 22 were killed. 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.