The warning comes after a series of bombings in Aleppo following the collapse of a week-long ceasefire in the country.
The United Nations' World Health Organisation says only a few dozen doctors remain in Aleppo to care for about a quarter of a million people.
And spokeswoman Fadela Chaib is calling for the safe evacuation of the sick and injured.
"Only about 35 doctors remain. All access routes to east Aleppo are closed. The remaining 25 health facilities are on the verge of complete destruction. WHO is calling for the immediate establishment of humanitarian routes to evacuate sick and wounded from the eastern part of the city."
The United Nations says more than a quarter of a million civilians are believed to be trapped inside the rebel-held area.
Intensive bombing over the past week has killed hundreds of people, many trapped under buildings brought down by bunker-busting bombs.
The offensive by the government, backed by Russia, is an attempt to drive out the rebels and retake control of Aleppo.
A volunteer organisation rescuing people trapped in bombed buildings in Aleppo says there is nowhere for civilians to hide or take cover in the city.
The Syria Civil Defence, also known as the White Helmets, says it has been witnessing what it calls a "ferocious campaign against Aleppo" in the past eight days.
Volunteer Ismail Alabdullah says he was unfamiliar with the powerful bunker-buster bombs until a few days ago.
"Actually, in the very beginning, when I felt a big explosion -- me and other civilians -- we didn't know the names of these rockets. After that, we learned, recently we learned, that these are bunker-buster bombs. What they do (is create) a big hole, because they can go through to under the ground, they can reach the basement. So, in this situation, no place is safe in Aleppo city. Many of the hospitals, many of the shelters, are underground, and now with these rockets, there is no safe place in Aleppo city."
A spokeswoman for the UN Special Envoy For Syria, Jessy Chahine, says the bombing has to end.
"The overwhelming danger to civilians in Aleppo today is from the sustained aerial bombardment with indiscriminate weapons in densely populated areas. This is, of course, inhumane, merciless, sinful and barbaric. It may amount to war crimes, and it must stop."
British foreign secretary Boris Johnson is calling for the same thing.
"The overwhelming responsibility for what is happening, the carnage that is being suffered by the people of Aleppo, lies with the (Bashar) Assad regime and their supporters, their puppeteers in Russia and, indeed, in Iran. And I think the most important thing now is that there should be a ceasefire and an end to the bombing."
Share
