SEOUL (Reuters) - Mr Otto Warmbier, the American university student being held by North Korea, was detained before boarding his flight to China over an unspecified incident that had taken place earlier in the trip at his hotel, his travel company told Reuters on Saturday (Jan 23).

North Korea's official KCNA news agency said on Friday that Mr Warmbier "was caught committing a hostile act against the state", which it said was "tolerated and manipulated by the US government".

Ms Charlotte Guttridge, a tour leader at Young Pioneer Tours and the only outside witness to Mr Warmbier's detention, said the 21-year-old University of Virginia student was not with other tourists when the events that appear to have prompted his arrest occurred.

"What happened, happened at the hotel and my belief is that Otto kept it to himself out of hope it might go unnoticed,"Ms Guttridge told Reuters.

Ms Guttridge and colleagues at Young Pioneer Tours declined to share further details of exactly what had taken place, citing the safety of their client.

Mr Warmbier had been staying at the Yanggakdo International Hotel when the incident that led to his arrest occurred. The Yanggakdo is a towering structure on an island in the middle of the Taedong river, which cuts through central Pyongyang.

China-based Young Pioneer Tours is a North Korea travel specialist that describes itself on its website as "an adventure tour operator that provides 'budget tours to destinations your mother would rather you stayed away from'".

During his five-day New Year's tour of North Korea, staff at Young Pioneer Tours said Mr Warmbier had acted normally, and was keen to see daily life in one of the world's most isolated countries, which is visited by around 6,000 Western tourists a year.

Ten other US citizens were on the tour.

"Throughout the trip, Otto behaved as a typical tourist - taking pictures, enjoying himself. We had no indication that anything untoward had happened until the airport," Ms Guttridge said.

When Mr Warmbier's group reached the airport, he appeared to have been purposefully delayed at immigration, Mr Troy Collings, director of Young Pioneer Tours, told Reuters.

As the tourists checked-in at the gleaming, recently renovated terminal, Mr Warmbier was taken aside by two airport officials and escorted into a small immigration room behind a wooden door to one side of the check-in area. "He was not dragged away and he wasn't yelled at," Ms Guttridge said.

As Ms Guttridge waited for Mr Warmbier to come out of the room, she instructed the rest of her tour group to board the North Korean Air Koryo flight bound for Beijing.

"When it became clear that he wasn't coming, I had to board the flight before it departed," said Ms Guttridge, who still had colleagues in Pyongyang with another group of tourists. "I was the last to board the flight."

As the Russian-made Tupolev airliner prepared to leave the terminal, an airport official boarded the plane and told Ms Guttridge that Mr Warmbier had been "taken to hospital".

Soon after, a North Korean contact passed on a message concerning Mr Warmbier's detention to Young Pioneer Tours founder Gareth Johnson, who was in Pyongyang with a separate group due to catch a train to the Chinese border.

"I stayed back when I heard Otto had been detained," Mr Johnson told Reuters. "It was an automatic response. I wanted to try and work out what the situation was and it was my hope that I would at least be able to speak with him."

Mr Johnson said his company was in contact with Mr Warmbier's family, US officials and the Swedish Embassy in Pyongyang, which represents US interests in North Korea.

Staff at the tour operator said as far as they knew Mr Warmbier had not been in possession of any religious or political literature. Foreign visitors have been detained in the past for attempting to distribute religious literature in the country.

The US and Canadian governments advise against travel there.

The US State Department, in a statement, said it was aware of reports that a US citizen had been detained in North Korea but gave no further details, citing privacy concerns.

Calls to the Warmbier family home in Cincinnati, Ohio, were not immediately answered on Friday and nobody answered when a Reuters reporter knocked on the door of the house.