Hill was paying a brief visit to Japan at the end of a trip that took him to Pyongyang, where he delivered a rare letter from US President George W. Bush pressing for progress in a six-nation nuclear deal.

Bush, in his first-ever direct communication with North Korean leader Kim Jong-Il, asked him to meet Pyongyang's promise to declare and disable all nuclear programmes by the end of the year.

Hill, the chief negotiator of February's disarmament-for-aid deal, said work to disable North Korea's Yongbyon nuclear plant was making headway and engineers were cleaning up contaminants before taking out the fuel.

"As important as the declaration is, it is also important to understand that the actual work on the ground in Yongbyon is proceeding, I think, very much on schedule," Hill told reporters at Narita airport near Tokyo.

"With regard to discharging fuel from the reactor, this is indeed a very important disabling step," he said.

"It was a step that could not be taken without efforts to clean up some contaminants, nuclear contamination," he said. "As I understand, all the equipment is in and the clean-up is almost... completed or soon to be completed."

"So I think we can expect discharging of fuel to get under way very soon if it has not gotten under way now," he said.

The North shocked the world with its first nuclear test in October 2006.

It agreed in February in the six-party talks to disable its plutonium-producing plants and declare all nuclear programs and facilities by year-end in return for major energy aid.

However, the agreement and the six-party process remained deadlock, with Hill returning to Washington without scheduling the next round of talks this week as hoped.

Analysts said that Bush's letter was aimed at deterring hardliners in both Washington and Pyongyang who would want to scupper the deal.

Bush, who addressed Kim in his message as "Dear Chairman," had once branded his regime part of an "axis of evil" and spoken critically about a previous deal with North Korea reached by Bill Clinton's administration.

Hill insisted that the February deal went even further than the 1994 agreement.

"All three of the Yongbyon facilities -- fuel fabrication, reactor and reprocessing facilities -- are undergoing disablement as we meet today at Narita airport," he said.

"The facilities were never disabled in the 1990s," he said. "Therefore, the six-party process has taken the nuclear issue further than it was ever taken in the 1990s."

"I think everyone should understand that these are important steps and we are pleased with cooperation we are getting," he said.

Bush also sent a letter to Japanese Prime Minister Yasuo Fukuda. Japan has been the most critical member of the six-nation talks due to a row over North Korea's past kidnappings of Japanese civilians.

In the letter, Bush spelled out the importance he attached to US-Japan cooperation, said Kenichiro Sasae, Japan's chief envoy to the talks, who met with Hill at the airport.

Sasae said he and Hill "reconfirmed our shared hopes both for progress in the six-party process and for a complete and correct declaration" by North Korea.

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