The US diplomat in charge of North Korea policy said Monday a speech by its foreign minister at the United Nations shows the country is moving further away from a return to six-country nuclear talks.

Glyn Davies, the special representative for North Korea policy, spoke with reporters in Beijing as he began a nearly week-long trip that will also take him to Seoul and Tokyo.

He cited "troubling further signs" that North Korea "is even more directly rejecting its responsibilities to live up to its obligation to denuclearise.

"We saw this again in a speech given by the foreign minister at the UN General Assembly in New York."

"It is essential that North Korea begin to take steps that move in that direction," he told reporters. "Instead, they're moving further and further away from that requirement."

In the first UN address by a top Pyongyang official in 15 years, North Korean Foreign Minister Ri Su Yong -- in a speech to the General Assembly on Saturday -- defended his country's nuclear weapons programme, which he said was necessary due to the "hostile policy" of the US.

The day Ri spoke, Chinese Foreign Minister Wang Yi in a speech at the assembly called for a resumption of talks on the North's programme as soon as possible.

China -- along with Russia, the US, North Korea, South Korea and Japan -- is a participant in the aid-for-denuclearisation talks, which have been stalled since 2009.

Davies said the purpose of his visit to the region was to stress "the importance of getting back to the necessity of denuclearisation" which he called the centrepiece of the talks.

He added that North Korea's repeated efforts to blame the US for its nuclear programme resembled a "broken record".

Davies' visit to Beijing comes amid North Korean state media reports that leader Kim Jong-Un is suffering a bout of ill health. Kim has not been seen in public since September 3.

The US envoy played down Kim's absence from the public eye.

"For us, what's important is not so much where the leader is but what North Korea's position is on the issues that are of deep concern to us," he said, declining to take part in what he described as "a game of 'Where in the World is Kim Jong-Un?'"

Davies also said the US was "very concerned" about three US detainees in North Korea -- Kenneth Bae, Jeffrey Fowle and Matthew Miller -- and was doing everything it could to gain access to them.

"I think it's unfortunate that North Korea continues to use these Americans ... as pawns," he said.

"They simply won't engage us," he said of North Korea. "It's very frustrating. It's unhelpful. And I think it puts the lie to the contention of the regime in Pyongyang that they want to have a relationship with us."