SEOUL - The top U.S. negotiator for North Korea went to the secretive state on Monday to visit an ageing nuclear complex at the heart of its atomic arms programme that Washington wants scrapped.

Assistant Secretary of State Christopher Hill, who flew to Pyongyang from a U.S air base in South Korea, will be in North Korea until Wednesday to help implement a deal where the impoverished state abandons its atomic ambitions in return for massive aid and an end to its status as an international pariah.

On arrival, Hill said he would first visit the Soviet-era Yongbyon nuclear complex and later meet the North's top nuclear envoy, Kim Kye-gwan, China's Xinhua news agency reported from Pyongyang.

Hill said in Seoul late last week: "I suspect, too, though that when I get there (Yongbyon) and see what we have done, I will also be struck by how much more will need to be done".

As part of the disarmament-for-aid deal, North Korea has started disabling three key facilities at Yongbyon, about 100 km (60 miles) north of Pyongyang: its sole working reactor, a plant that makes nuclear fuel, and another that turns spent fuel into plutonium.

The deal North Korea reached with the United States, South Korea, China, Japan and Russia also commits it to provide a complete inventory of its nuclear arms programme by year's end and answer U.S. suspicions it was secretly enriching uranium for weapons.

If Pyongyang complies, Washington has pledged to start the process of removing it from a State Department list of countries that sponsor terrorism.

A team of U.S. nuclear experts has been at Yongbyon for the past several weeks to disable the facilities. U.S. and South Korean officials said North Korea had been cooperating.

After leaving North Korea, Hill is due to head to Beijing for another round of six-way nuclear talks.

The next session is expected to outline steps to dismantle the North's nuclear facilities and the rewards Pyongyang will receive for compliance.