US politician Bill Richardson and Google chairman Eric Schmidt visited North Korea's largest library and paid respects to its late leaders on the third day of their visit Wednesday, the state news agency said.

Richardson has described the trip, which attracted criticism from Washington, as a "private humanitarian mission" and the secretive nation's official media has released few details about their activities.

Richardson, a former New Mexico governor and an ex-ambassador to the United Nations, has been to the North several times in the past two decades and has helped negotiate the release of detained US citizens.

Kenneth Bae, an American of Korean descent, is being held in North Korea and his son contacted Richardson to ask for his help, the latter said last week.

The North has in the past handed over detainees to high-profile delegations and some observers suggested it may have requested Schmidt's participation in this case.

But the US State Department has voiced concern about the trip, saying it was ill-timed in the wake of Pyongyang's widely condemned rocket launch last month.

The Korean Central News Agency (KCNA), which describes the visitors as a Google delegation, said they visited reading and lecture rooms at the Grand People's Study House and were briefed on scientific and technological studies.

The study house is a massive central library and a prominent landmark in the showpiece capital Pyongyang.

Another stop was the electronic library at the computer centre of Kim Il-Sung University.

The delegation also visited the Kumsusan Palace of the Sun, a mausoleum housing the bodies of late leaders Kim Il-Sung and Kim Jong-Il.

"The members of the delegation paid high tribute to the statues of the peerlessly great men," said KCNA.

Richardson and Schmidt were to fly back to Beijing on Thursday and give a press conference on arrival.

Richardson said before departure he hoped the trip would be "positive" and dismissed US concerns, saying it had already been postponed once at Washington's request and the State Department should not be "nervous".

Bae, who was arrested in November, entered the country as a tourist, according to the North's official news agency which said he had admitted committing a crime against the state.