A US envoy Saturday stepped up pressure on North Korea to return to nuclear disarmament negotiations, voicing hope they will resume "fairly soon."

Stephen Bosworth, on a tour of Asia aimed at reviving the stalled six-party dialogue, said all the member states except North Korea "are prepared to move very quickly."

"We will hope that the sixth -- that is to say the DPRK (North Korea) -- will also decide to move ahead very quickly," the US special envoy to North Korea told reporters in Tokyo.

"There is a strong desire to get back to the table and begin serious work on the very important issues that we face," Bosworth said.

"I hope that in the not distant future but fairly soon we will see a resumption of the talks," Bosworth said. "For our part, we are ready to move in on very short notice."

China, which has hosted the disarmament talks and maintained close ties with North Korea, has continued "extensive contact" with Pyongyang to persuade it to return to the talks.

"However, what we have not yet been able to agree on is... when the six-party process will actually resume," he said, reiterating that it was up to North Korea when the dialogue restarted.

The North has two conditions for returning to talks: the lifting of UN sanctions and a US commitment to discuss a formal peace treaty on the Korean peninsula.

Seoul, Washington and Tokyo say the North must return to the negotiating table and make substantial progress toward denuclearisation before other matters are discussed.

There are no clear signs so far that the North is willing to drop its conditions for returning to the talks, which groups the two Koreas, the United States, China, Russia and Japan.

Bosworth's remarks came after US Secretary of State Hillary Clinton said on Friday that the United States is "encouraged by signs of progress" toward the resumption of six-party talks.

Clinton, who met South Korean Foreign Minister Yu Myung-Hwan on Friday in Washington, said the United States is now "closely cooperating" with South Korea and Japan as well as China.

Bosworth, who visited Pyongyang last December, also hold reporters that he has no immediate plan to hold separate talks with North Korean officials, saying a resumption of the six-way talks is his top priority.

Washington has "no philosophical objection to meeting bilaterally with the DPRK, but our objective remains to resume the multilateral dialogue as soon as possible," he said.

On Friday, Bosworth met Japanese Foreign Minister Katsuya Okada and talked about a Chinese proposal for a breakthrough in the stalled dialogue, Japanese media reported without unveiling details of the plan.

But Okada was cautious about the prospects for swift progress.

"We should not think about the situations too optimistically," Okada told reporters.

"It's important not just to resume the six-party talks but reach a conclusion on nuclear disarmament through the talks," he added. "We need patience to achieve the goal."