China has played down North Korea's failure to meet a deadline on declaring its nuclear programme.
North Korea started disabling Yongbyon in October
"The pace is faster in some areas and slower in others. This is normal," a foreign ministry spokeswoman said.
Pyongyang had promised to declare all its nuclear programmes and disable its main reactor by the end of 2007, but neither deadline was met.
The pledge was part of a deal agreed in February under which North Korea would become nuclear-free in return for aid.
The US recently expressed doubt that North Korea would ever give a complete declaration of its nuclear activities.
"We don't have any reason to believe that they won't, but we are sceptical, given the length of time it's taken," White House spokeswoman Dana Perino said on Wednesday.
In an effort to keep the nuclear deal on track, the US chief envoy to North Korea, Christopher Hill, will travel to Asia later this week for talks on the issue.
Deputy Secretary of State John Negroponte has also announced plans for a trip in mid-January.
North Korea agreed to the disarmament-for-aid deal during six-party talks in February 2007 - involving delegates from China, the US, Japan, Russia and the two Koreas.
Since then, it has closed its main reactor at Yongbyon, and efforts are under way to disable it.
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The process was due to be completed by the end of December, but there has been a delay, which is reportedly due to technical reasons.
However, it is the lack of a written declaration providing a complete account of all Pyongyang's nuclear activities that is of more concern to the international community.
In particular, the US wants to know how much plutonium has been produced by North Korea, and also see evidence that there is no secret programme for uranium enrichment for weapons purposes.
North Korea denies the existence of any such programme, and has attributed the delay in its declaration to a slow delivery of promised aid.