The diplomats are hoping to use a private conference in Tokyo to scratch out progress ahead of a visit to Washington next week by Hu Jintao, the Chinese president.

The conference, organised by the University of California in San Diego, is the first meeting of the six chief envoys involved in the talks since negotiations broke down without agreement in November last year.

Wu Dawei, the Chinese envoy and host of the six-party talks, said he believed there was "still some time left" to reach an agreement in Tokyo.

His comments came amid reports that he had extended his stay in the Japanese capital by a day to push for a breakthrough.

However, North Korea remained firm on its demand that the US end sanctions before it would return to the talks.

Meanwhile, the US delegation said it would not meet the North Koreans until they agreed to resume talks without preconditions.

'Set to go'

"This is not about talks, this is really about action"

Christopher Hill,
US assistant secretary of state for East Asia

Christopher Hill, the US assistant secretary of state for East Asia, said: "This is not about talks, this is really about action, and the action is they need to join the six-party process. We are ready. I've got a suitcase all set to go.

"We have done our homework. They need to do their homework, and then we need to get on with it."

Speaking on Tuesday, Hill said he had had an "excellent" meeting with Wu, adding that Beijing was committed to resolving the standoff through dialogue.

China has hosted five rounds of six-nation talks since 2003 directed at disarming North Korea, which declared last year it had nuclear weapons.

North Korea has refused to return to the negotiations - involving China, Japan, the two Koreas, Russia and the US - since Washington blacklisted a bank for allegedly counterfeiting dollars and money-laundering.

The ban against US institutions dealing with the Macau-based bank is believed to have badly hit the fragile North Korean economy, which is heavily dependent on Chinese aid.