* UN says no harm must come to male detainees
* Evacuation, aid halted until Wednesday morning
* Syrians evacuated describe severe hunger in besieged Homs (Adds halt to evacuations, governor says 100 men released)
GENEVA, Feb 11 (Reuters) - 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 on Tuesday.
The men, deemed to be of fighting age by the Syrian authorities, were among 1,151 who left the besieged rebel-held Old City of Homs during an agreed ceasefire that was extended for another three days, until Wednesday.
The U.N.-brokered "humanitarian pause" between 6 a.m. and 6 p.m. has also llowed 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.
Homs governor Talal al-Barazi said around 100 men had been questioned and released, but the United Nations has so far only reported the release of 41 men.
The men have been questioned in a school, under the "general monitoring" of protection staff from the U.N. refugee agency UNHCR and the U.N. children's agency, UNHCR spokeswoman Melissa Fleming told a briefing in Geneva.
"We know every person that is there. We are speaking to them separately ... But these are interviews that the U.N. is not necessarily privy to. These are security screening interviews," Fleming said.
"Mostly what we're asking about is we're concerned in general about how they're faring, what's their health situation, what are their concerns," she said.
The U.N. was also asking detainees about the humanitarian situation inside Old Homs, she said, to better inform aid workers going back in for evacuations and aid distributions.
Aid and evacuations operations were halted on Tuesday afternoon for logistical reasons, the governor said, but will continue again on Wednesday morning at 9:30 a.m.
"We've delayed today's operations until tomorrow, when we will have prepared a new corridor into the area, which is closer to the civilians there, especially for the elderly ... It is difficult for some of them to walk," he told Reuters by telephone.
The ceasefire deal originally stipulated that only women, children and men over 55 years of age would get safe passage.
But on Sunday, Barazi said anyone could leave, though men aged between 15 and 55 would be questioned and put through a "judicial process", which could include an amnesty.
Opposition activists have expressed concern that some Homs evacuees, particularly men, could encounter dangers such as those faced by residents who fled the besieged rebel-held town of Mouadamiya, near the capital, Damascus, in October.
Security forces detained dozens of men, many of whom have not been freed.
"ONE SPOON OF BULGUR"
Rupert Colville, spokesman for the U.N. 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, torture and humiliating 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 Old Homs ceasefire has been the first tangible result of the "Geneva 2" peace talks, in their second round this week.
The evacuees included five women in the late stages of pregnancy, including one who was in labour and gave birth in hospital shortly afterwards, said World Health Organization spokeswoman Fadela Chaib.
Many people had skin diseases, she said.
Those coming out were very weak with obvious signs of malnutrition
"A man said he survived for a week with one spoon of bulgur," said World Food Programme spokeswoman Elisabeth Byrs, adding that the small amount of bulgur wheat available in the besieged area was infested with insects.
"They said that daily they survived on leaves, grass, olives, and sometimes some wheat flour appeared and they were mixing flour with water to make a kind of bread."
Syria's nearly three-year-old conflict has killed more than 130,000 and forced six million to leave their homes. Many live in desperate conditions, with hunger an increasing problem.
The United Nations says it does not know how many people are still in Old Homs, but some estimates suggest the area has about 2,500 residents - a small percentage of the more than quarter of a million the U.N. estimates are trapped.
Assad's forces have often used sieges to choke rebel-held areas. Rebels are increasingly doing the same. (Reporting by Stephanie Nebehay and Tom Miles; Writing by Erika Solomon; Editing by Ralph Boulton)