Syrian authorities have detained 336 men who left Homs and are still questioning most of them without direct supervision by any neutral third party, the United Nations said.
The men, deemed to be of fighting age by the Syrian authorities, were among 1,151 who left the besieged Old City of Homs during an agreed ceasefire that was extended for a second three-day period on Monday.
Forty-one of the men have been released, but the rest are being questioned in a school, under the "general monitoring" of UN protection staff from the UNHCR and UNICEF.
UNHCR spokeswoman Melissa Fleming said: "We're in the facility, we know every person that is there.
"We are speaking to them separately themselves. But these are interviews that the UN is not necessarily privy to.
"These are security screening interviews."
Rupert Colville, spokesman for the UN human rights office, said any evacuee, including those who had laid down their arms, must be protected from acts prohibited under international law, including cruel treatment and degrading treatment.
"We are also deeply concerned to learn that a number of boys and men and their families were seized by the authorities as they left the besieged area. It is essential that they do not come to any harm," he said.
The UN-brokered "humanitarian pause" between 6am and 6pm has enabled aid to get into the old quarter of Homs, which has been surrounded by President Bashar al-Assad's forces for more than a year and a half.
Meanwhile, Russian Foreign Minister Sergei Lavrov said this morning that a draft UN Security Council resolution aimed to boost aid access to Syria was one-sided and detached from reality, the Interfax news agency reported.
Mr Lavrov said those behind the draft were seeking to mount pressure on Russia and China by discussing their proposal with the media.
He said: "The ideas that were shared with us by those initiating this process ... are absolutely unacceptable and contain an ultimatum for the government, that if they don't solve all this in two weeks then we automatically introduce sanctions."
Mr Lavrov urged the Security Council to instead ready a resolution condemning "terrorist activity" in Syria.
Meanwhile, Syria's government and opposition are meeting for their first face-to-face talks this month.
The session in Geneva is being chaired by UN-Arab League envoy Lakhdar Brahimi.