Hundreds of men are feared missing after leaving rebel-held areas of eastern Aleppo as government forces tightened their grip on the besieged city, the United Nations warned Friday.

“While it is difficult to establish the facts in such a fluid and dangerous situation, we have received very worrying allegations that hundreds of men have gone missing after crossing into government-controlled areas,” Rupert Colville, a spokesman for the U.N. High Commissioner of Human Rights, said in a statement.

Residents gather at a checkpoint manned by pro-government forces in the Maysaloun neighborhood of the northern embattled Syrian city of Aleppo on Thursday. YOUSSEF KARWASHAN / AFP - Getty Images

Big advances have been made by Russia-backed Syrian troops in recent weeks. The Russian military said Friday that 93 percent of Aleppo is now under the control of the Syrian army.

Aleppo, Syria’s largest city before the war, has been divided between rebel and government forces since 2012. Intensifying government efforts to recapture the city have forced civilians and opposition fighters to flee.

Read: Almost 150 People Evacuated From Aleppo's Old City

Colville added that family members had reported losing contact with many men, all of whom are aged between 30 and 50 after they fled opposition-controlled areas of Aleppo between a week and ten days ago.

"Given the terrible record of arbitrary detention, torture and enforced disappearances by the Syrian government, we are of course deeply concerned about the fate of these individuals," he added.

The U.N. also raised worries over reports that armed groups could be treating citizens like "pawns" and preventing them from fleeing the besieged and rapidly shrinking rebel-held enclave in Aleppo.

He mentioned reports that the Fatah al-Sham Front (formerly al-Nusra Front) and the Abu Amara Battalion allegedly abducted and killed an unknown number of civilians who requested the armed groups leave their neighborhoods to spare the lives of civilians.

The UNHCR estimates that there are 100,000 civilians in areas under the control of opposition groups in eastern Aleppo, with another 30,000 believed to have fled to areas under Government control.

Secretary of State John Kerry spoke of the crisis during a visit Friday to the U.S. Embassy in Paris, telling staffers that the Obama administration continued to seek a resolution to what he called "the worst catastrophe since World War II."

"We are working hard with people that we even have disagreements with in order to see if we can find a way in the name of humanity and decency to be able to protect those lives and try to separate combatants and move the process forward," Kerry said.

The next step will be a Saturday meeting with Russian diplomats in Geneva, Kerry said.

He acknowledged that an endless series of meetings seemed not to be producing results.

Syrian residents of the newly retaken eastern neighbourhoods of Aleppo walk through the Bab al-Hadid district as they go back home under supervision of the Syrian pro-government forces on Thursday. GEORGE OURFALIAN / AFP - Getty Images

"But what am I supposed to do? Go home and have a nice weekend in Massachusetts while people are dying? Sit there in Washington and do nothing? That’s not the way you do business," he said. "That’s not what the United States does, it’s not what people of decency do, and that’s not what we do if we keep faith where those before us have always proved that you’ve got to keep."

Lt. Gen. Sergei Rudskoi of the Russian Armed Forces said Friday that more than 10,500 people, including 4,500 children, had been led out of areas controlled by rebels over the past 24 hours.

Rudskoi also stated that a total of 1,096 militants had voluntarily left Aleppo, with 953 granted amnesty and the rest are undergoing checks.

Meanwhile, The Associated Press reported Friday that residents and medical workers in eastern Aleppo said there had been no letup in the bombardment.

On Thursday, the “White Helmets” first responders group urged the International Committee of the Red Cross (ICRC), the United Nations and the members of the Security Council to protect its members in east Aleppo.

“If we are not evacuated our volunteers face torture and execution in the regime’s detention centers,” the group said in a statement, adding that the regime and its allies falsely claim “White Helmets are in fact affiliated with radical extremist groups.”

The fall of Aleppo would be a major victory for the President Bashar al Assad who is looking to wrestle back control of the country that has descended into chaos since he cracked down on pro-democracy contests in 2011.

The ensuing civil war has killed hundreds of thousands of people, seen ISIS take control of large parts of the country and created a refugee crisis that has been felt as far away as Europe.