North Korea began disabling its plutonium-producing reactor and other related facilities last month with US experts present.
 
The process is a key step in a six-nation deal aimed at persuading Pyongyang to abandon its nuclear programme in return for economic aid and other benefits.
 
A team of US nuclear experts has been at the Yongbyon complex - located about 100km north of Pyongyang - for several weeks oversee and advise on disabling the plant.
 
North Korea had promised to complete the measures by the end of the year but Chun Yung-woo, the chief South Korean nuclear envoy, said last week it would take longer to remove about 8,000 spent fuel rods from the reactor.
 
Declaration
 
A crucial issue during Hill's three-day trip is expected to be another North Korean pledge to provide a complete inventory of its nuclear programme by the end of the year and answer US suspicions it was also secretly enriching uranium for weapons.
 
Pyongyang is expected to fully declare its
nuclear programme by year's end [AP]
Hill, who flew to the North from a US air base in South Korea after a four-day visit, said last week that the North was finalising the declaration of its nuclear programmes and that he would talk about the document with North Korean officials.
 
Besides the plutonium programme, Hill has said the list should also give a "complete understanding" of a suspected uranium programme to build bombs, even if it is no longer active.
 
The US envoy has also stressed the need for the North to include its proliferation efforts in the list, amid suspicions it may have helped Syria build a nuclear plant.
 
The content of the declaration could indicate whether the North is willing to go all the way to full denuclearisation, or whether it plans to keep its stockpiled material after shutting down ageing plants.
 
If the North fulfils its commitment, the US has pledged to start the process of removing it from its list of countries that it deems sponsors of terrorism.
 
Hill last visited North Korea in June and was at that time the first senior US official to do so in more than four years.
 
After his current trip, which ends on Wednesday, Hill is expected to join envoys from the two Koreas, China, Russia and Japan in Beijing to resume six-nation talks set to outline steps to dismantle Pyongyang's nuclear facilities.