Former New Mexico governor Bill Richardson and Google chairman Eric Schmidt arrived Monday in North Korea, where a US citizen is being held awaiting trial, for what the politician called a private trip.

"This is a private humanitarian mission, not connected to the US government," said Richardson, also a former US ambassador to the United Nations, at Beijing's airport before travelling to the secretive nation.

"We're going to be in Pyongyang, probably for two and a half days. We may go outside the city. We will find out when we arrive," he said.

The North's official KCNA news agency announced their arrival in Pyongyang as "a US Google delegation headed by former governor of New Mexico Bill Richardson".

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.

North Korea has in the past agreed to hand over detainees to high-profile delegations led by the likes of former US president Bill Clinton, 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.

State Department spokeswoman Victoria Nuland said Monday Washington's opinion had not changed, but hinted the two men might be asked to brief US officials about the trip afterwards.

"We are always open to hearing from Americans who've been in North Korea," she said.

Richardson has been to North Korea a number of times in the past two decades and has been involved in negotiating the release of US citizens held in the isolated country.

He told CNN on Friday that he expected to meet several senior officials in North Korea, though talks with leader Kim Jong-Un were "very doubtful".

He added that 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".

Both he and Schmidt would be travelling as private citizens, representing neither the US government nor Google, he said.

Also on the trip were Richardson's longtime aide on North Korea, K.A. "Tony" Namkung, Jared Cohen, director of Google Ideas, a think-tank run by the California-based Internet giant, and some staff, according to a statement from Richardson's office.

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.

Nolan Barkhouse, spokesman for the US embassy in Beijing, said Sunday that Richardson's trip was unrelated to the authorities in Washington.

"They will not be carrying any messages from the US government," Barkhouse told AFP, adding: "They will not be accompanied by any US officials."

Asked about the trip, a spokesman for China's foreign ministry said only: "We have noticed relevant press reports."

Richardson last visited the North in 2010 when he met its chief nuclear negotiator to try and ease tensions after the country shelled a South Korean border island.

North Korea last month angered the US and others by launching a long-range rocket. It said the purpose was to put a scientific satellite into orbit but Washington and other nations called it a disguised ballistic missile test.