Civilians are trying to flee a tightening siege in the rebel-held parts of Syria's northern city of Aleppo, as government forces and their Russian allies continue to seize territory while trying to take full control of the city.

They're taking aim at the final major urban area still held by rebel forces.

A new U.N. report suggests that hundreds of men have disappeared upon entering government-controlled areas of the city. The men are being separated from women and children, the report states, and "family members say they lost contact with the men, between the ages of 30 and 50, after they fled opposition-controlled areas of Aleppo around a week or ten days ago."

Many other residents have chosen to stay or are trapped in the remaining few neighborhoods that rebels have held, as NPR's Alison Meuse reports on Morning Edition. She reached a dentist named Salem Abu al-Naser who is trapped in one of the last rebel-held neighborhoods, which he says is being pummeled with artillery, rockets and vacuum missiles. "I don't even know how to describe the kinds of things that are landing on us," he tells Alison.

"These are people who love life. It's not like Aleppo is just full of a bunch of fighters or radicals," Abu al-Naser tells Alison, saying that in many cases they stay because they don't want to be homeless.

"We're talking about regular people sitting in their homes," he says. "They don't want to live in refugee camps; they don't want to go to Europe. They don't want to flee."

You can hear Alison's full report here:

Meanwhile, U.N. negotiators are frustrated by the lack of progress in talks on how to help the civilians who are trapped. "[T]he Member States that are supposed to help us get access [are] poles apart on what is happening in Syria," United Nations Senior Adviser Jan Egeland told reporters on Thursday. Here's more from the U.N. on why the process is so difficult:

" 'I have never, in my many, many years of humanitarian negotiations, been in as difficult negotiations and as frustrating talks that produced nothing in spite of thousands of contacts with all of the parties, and it is with bitterness and frustration that we have to report that we have not been able to evacuate even the wounded,' he announced."In order to provide humanitarian assistance, cooperation is necessary from the Government of Syria, the Russian Federation, various armed opposition groups, opposition health directorates, and others who must all agree on where, how, what conditions, guarantees, protection standards, and logistics." 'If only one disagrees,' he said, 'the whole thing fails.' "

The Red Cross and the Syrian Arab Red Crescent were able to evacuate about 150 civilians from a hospital near the front line, the groups said in a statement. Most of them "were disabled or in need of care," according to the statement. But rescue workers didn't get there in time for some of those staying at the center:

"Tragically, for some, the operation came too late: 11 people died before the SARC and ICRC team were able to reach the centre. They were either caught in the crossfire or died because they did not have access to the right medications. The SARC also evacuated their human remains, so they can be properly identified and returned to their families, and accorded the appropriate funeral rites."

There are about 100,000 civilians still in rebel-controlled areas, including about 500 people with urgent medical needs that require urgent evacuation, U.N. Human Rights office spokesman Rupert Colville told reporters.

Today, the U.N. General Assembly voted to call for a cease-fire in Aleppo. NPR's Michele Kelemen tells our Newscast unit that the nonbinding resolution passed, 122-13.

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