"Utmost on our mind is the need to address the very concerning medical situation" in the east of Aleppo, UN deputy envoy for Syria, Ramzy Ezzeldin Ramzy, told reporters in Geneva.
"Medical evacuations are urgently needed," he stressed, adding that "probably hundreds" of people needed to be urgently evacuated from the war-ravaged city.
His comments came a day after two of the largest hospitals in the city's east were bombed, prompting UN Secretary General Ban Ki-moon to describe that attack as a war crime.
Ramzy warned that medical supplies were running dangerously low and only around 35 doctors remained in eastern Aleppo, where an estimated 250,000 people have been under siege by government forces since early September.
"As many as 600 wounded cannot be provided with adequate treatment," Ramzy said.
He also cautioned that "food stocks are running low", with many bakeries closed and only 14,000 food-aid rations remaining.
With each of those rations enough to feed five people, that would be sufficient for 70,000 people, or only about a quarter of the population, according to the UN's World Food Programme.
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"We hope it will be possible to create conditions for (aid) deliveries to be made. The UN continues to be ready to deliver humanitarian assistance including medical supplies as soon as possible," Ramzy said.
Speaking after a meeting of the UN-backed humanitarian taskforce for Syria, Ramzy said the discussions had been "long and difficult" and overshadowed by the situation in Syria's second city.
He said the UN had appealed to taskforce co-chairs Washington and Moscow to help clear the way for desperately-needed aid to go into Aleppo and other areas.
The United States and Russia have meanwhile been busy trading blame over the collapse of a truce they negotiated on September 9, with Washington harshly criticising Moscow's participation in the Aleppo offensive by forces of Syrian President Bashar al-Assad.
"The bombing must stop. Civilians must be protected. And the cessation of hostilities must be restored," Ramzy insisted.
The UN's top envoy for Syria Staffan de Mistura told AFP Thursday that there was little prospect of restarting long hoped-for peace negotiations in light of the situation on the ground.
Dozens of civilians have been killed, residential buildings have been reduced to rubble and residents of east Aleppo are facing severe shortages.
The UN children's agency UNICEF said at least 96 children have been killed and 223 wounded since Friday in eastern Aleppo.