US diplomat Chris Hill has arrived in Seoul ahead of a visit to North Korea to try to save a crumbling disarmament-for-aid deal and prevent Pyongyang from rebuilding its ageing nuclear plant.

North Korea has threatened to restore its nuclear plant that makes bomb-grade plutonium, in anger at not being dropped from a US terrorism blacklist and by Washington's verification demands.

The Yongbyon plant was frozen and has been in the process of being disabled under the deal.

"Let's see if we can come up with measures that will allow us to verify their declaration," US Assistant Secretary of State Hill told reporters at Incheon airport near Seoul.

Mr Hill is set to travel by road to North Korea on Wednesday on a journey of about three hours that takes him across the heavily armed border. He did not say when he planned to return.

The International Atomic Energy Agency said last week that the North was expelling UN monitors from its Soviet-era nuclear plant and plans to start reactivating it in days, rolling back a disarmament-for-aid deal and putting pressure on Washington.

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