The US diplomat responsible for North Korea said Friday that Pyongyang is proceeding with its nuclear programme and showing no clear sign of steps that would permit the resumption of international talks on ending it.

The negotiations, known as the six-party talks, began in 2003 and consist of five countries -- China, Japan, Russia, South Korea and the United States -- trying to convince North Korea to drop its atomic development in exchange for aid and other incentives.

Pyongyang walked out of the talks in 2009, three years after carrying out its first nuclear test, and has subsequently detonated two more devices underground.

North Korean officials have repeatedly said the secretive regime is willing to resume the discussions unconditionally, but the US and others are demanding that it concretely demonstrate its sincerity first.

"There's very strong consensus among the five parties that in order for us to have any chance of success in nuclear negotiations we need a very strong commitment from the North Koreans that in fact they would be a serious negotiating partner if and when negotiations resume," Sung Kim told reporters in Beijing.

"And that's what we're waiting to see from the North Koreans," added Kim, Washington's special representative for North Korea policy.

"We are concerned that North Koreans are continuing to pursue their nuclear programme, improve their nuclear and missile capabilities."

Kim, a former US ambassador to South Korea who assumed his current post in early November, spoke at the conclusion of a trip that also included stops in Tokyo and Seoul.

- 'Potential for change' -

In September 2005 Pyongyang agreed in writing to stop the programme. But as well as the nuclear test explosions it has also conducted ballistic missile launches, developments which led to a series of censures and sanctions by the UN Security Council.

Kim said that Pyongyang's release in October and November of three US citizens it had been holding was a welcome development, but it had not resulted in any visible change in the country's attitude toward the nuclear issue.

"We're all waiting to see some clear indication from the North Koreans that they're willing to work with us on preparations for the resumption of the talks," he said.

"Very generally speaking, it would be hard for us to negotiate with the North Koreans while they're continuing to carry out nuclear activities."

Kim said he had "very productive discussions" with Chinese officials and called on Beijing to continue using its "considerable amount of influence and leverage" over Pyongyang to get it back to the negotiating table.

He also praised China's approach to making sanctions against North Korea work.

"I believe Chinese are taking that obligation very seriously and we expect that sanctions enforcement to continue," he said.

China, which fought alongside North Korea in the 1950-53 Korean War, has long been considered Pyongyang's diplomatic protector, though in recent years has taken a harder approach, on issues such as sanctions, while forging increasingly friendly relations with South Korea.

Wang Hongguang, a retired Chinese People's Liberation Army general, wrote in the China-based Global Times newspaper this month that Beijing will not step in to save North Korea if the regime collapses or starts a war, but also won't walk away from it.

China will neither "court" nor "abandon" North Korea, he wrote in the tabloid, which is close to the ruling Communist Party. "This should be China's basic attitude."

US director of National Intelligence James Clapper, who flew to North Korea on a secret mission to free two of the Americans in November, expressed optimism after its conclusion.

"I do think there is the potential here for change and dialogue in the future," he told The Wall Street Journal in an interview, recounting his interaction with a North Korean official during the visit.