"Aleppo is now a synonym for hell," Ban said in his final news conference before he steps down as UN chief on December 31.
The Syrian government suspended the evacuation of civilians and fighters from Aleppo today, leaving thousands of people trapped and facing an uncertain fate.
The opposition accused the government of suspending the operation in a bid to secure the evacuation of residents from two villages under rebel siege.
It is unclear how many people remain in eastern Aleppo, with tens of thousands fleeing to territory held by the government or Kurds in recent days but perhaps tens of thousands still inside.
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The United Nations is "urging the parties to take all necessary measures to allow safe resumption of this evacuation process," he added.
Under a deal reached with Syria and backed by Russia and Turkey, evacuations began from the last remaining pocket of rebel opposition in Aleppo on Thursday, but were halted in the early hours on Friday.
Thousands of people were allowed to leave the city, including 194 patients who were taken to hospitals in Idlib, western Aleppo province and Turkey for treatment, said Ban.
"The immediate priority is to save lives, to stop the massacres and to avoid a new Srebrenica," French Ambassador Francois Delattre said, referring to the massacre of Bosnian Muslims at the hands of Bosnian Serbs during the Balkan wars.
The fall of Aleppo will hand President Bashar al-Assad his biggest victory in the nearly six-year war that has killed more than 300,000 and displaced half of the country's population.