Progress on disabling North Korea's nuclear site is "going well," according to the top US nuclear negotiator.
North Korea is removing 8,000 fuel rods from the Yongbyon reactor
Assistant Secretary of State Christopher Hill visited the Yongbyon reactor on Monday to assess the work.
But he said Pyongyang must continue its efforts to produce a complete declaration of its nuclear programme by the end of the year.
North Korea has pledged to bring an end to its nuclear programme in exchange for aid and diplomatic concessions.
Mr Hill told reporters in Pyongyang that he was given a full tour of the nuclear complex and had a "good two days of talks" with North Korean officials.
"The disablement activities are going well and on schedule," he said.
"I'm satisfied with the results. But we have to keep working because we have more to do to meet our deadlines."
Mr Hill also held a meeting with his North Korean counterpart, Vice-Foreign Minister Kim Kye-gwan.
North Korea is in the process of removing 8,000 spent fuel rods from its only nuclear reactor, under the supervision of American experts.
Mr Hill is the highest profile US diplomat to visit North Korea
Pyongyang agreed to do so at six-party talks in October, in exchange for aid packages and an improvement in its diplomatic relations with other countries.
The country has also pledged to submit a complete documentation of all its nuclear activities by the end of the year, including any details of its alleged secret nuclear weapons programme.
Mr Hill said he was confident that Pyongyang would submit a complete list on time.
"I think the DPRK (Democratic Peoples' Republic of Korea) is working very hard for the declaration and we had some discussions about that ... so I'm assured they will be prepared to meet the deadline," he told Xinhua news agency.
The US has suggested that if the declaration is a success, they will consider removing North Korea from a list of countries it says supports terrorism.
However, a Japanese parliamentary commission has passed a resolution urging the US not to do so until Pyongyang addresses the issue of abducted Japanese citizens.
Pyongyang kidnapped several Japanese nationals in the late 1970s and early 1980s to train as spies. It has returned some but Tokyo believes that more remain.
A new round of six-party talks - involving North and South Korea, the US, China and Russia - was due to start in Beijing this week but has been called off.
The Associated Press news agency quotes the US State Department as saying this was due to scheduling problems and that talks would be held in the "near future".