The top U.S. nuclear envoy for North Korea said disablement of that country's atomic program is on schedule and that the removal of fuel from its key facility is under way.

Christopher Hill, who delivered an unprecedented letter from U.S. President George W. Bush to North Korean leader Kim Jong Ill during his visit this week in Pyongyang, also urged the North to provide a "complete and correct" disclosure of its nuclear programs.

"As important as the declaration is, it's also important to understand that actual work is on the ground in Yonbyong and is proceeding very much on schedule," Hill said during a transit in Japan following a three-day North Korea visit to inspect the North's nuclear disablement process at the main facility in Yonbyong.

Under a six-nation agreement in February, North Korea was promised to receive energy assistance and political concessions including its removal from a U.S. terrorism blacklist, in return for disabling its nuclear programs.

Efforts to produce that draft declaration by a year-end deadline appeared to have hit a snag, but Hill reiterated Friday he still has hope that North Korea will produce a substantive document.

"I do believe they can have a draft produced before the end of the year," he told reporters after meeting with his Japanese counterpart, Kenichiro Sasae, at the Narita International Airport near Tokyo before flying back to Washington.

The two envoys shared "strong hope" they would obtain North Korea's full declaration, and agreed to cooperate to achieve the goal, Sasae said.

Hill said work to disable North Korea's Yongbyon nuclear plant "are moving quite on schedule" and workers are cleaning up contaminants before discharging the fuel from the reactor, considered a key disabling step.

"As I understand, all the equipment is in and the cleanup 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," Hill said.

Potential complications include North Korea's alleged uranium enrichment program and the true amount of separated plutonium that it declares. The U.S. accused North Korea in late 2002 of seeking to secretly enrich uranium in violation of an earlier disarmament deal, sparking the latest nuclear standoff.

In his letter, Bush said North Korea's declaration must be "complete and accurate if we are to continue our progress," according to an excerpt of the Dec. 1 letter obtained by The Associated Press.

Bush also sent similar letters to Russia, China, South Korea and Japan — the other participants in six-nation North Korean disarmament talks that have dragged on for four years.

Pyongyang conducted its first nuclear test in October 2006, prompting international condemnation and sanctions.

Relations between the U.S. and North Korea have improved in recent months after Pyongyang began disabling its reactor, which was shut down in July, and two other facilities.