Outgoing U.N. Secretary-General Ban Ki-moon on Friday referred to Aleppo, Syria, as a "synonym for hell" during his final news conference before stepping down at the end of the year.
"We have collectively failed the people of Syria. Peace will only prevail when it is accompanied by compassion, justice and accountability for the abominable crimes we have seen," Ban said.
Earlier Friday, the Syrian government suspended an evacuation operation to move civilians out of eastern Aleppo, leaving thousands of people trapped in the besieged city.
Rebels accused the government of calling off the evacuation to put pressure on them to allow civilians to evacuate two villages currently under siege by rebels. The government said it called off the evacuation because rebels opened fire on a convoy of vehicles.
Ban called on the Syrian government to continue the evacuation effort. The U.N. is "urging the parties to take all necessary measures to allow safe resumption of this evacuation process," he said.
In his farewell address to the U.N. press corps, Ban covered a variety of topics, including the conflict in South Sudan, Israeli settlements in the West Bank and the signing of the Paris climate agreement, which he called a "precious achievement that we must support and nurture."
"Difficult as it may sometimes be, international cooperation remains the path to a more peaceful and prosperous world," he said.
Ban, who has led the U.N. for nearly a decade, will hand over the reins to former Portuguese Prime Minister Antonio Guterres on January 1. Guterres was sworn in as Ban's successor on Monday.