The state department refused to confirm Hill was going to Pyongyang but said he was traveling to Seoul, the South Korean capital, on Monday.

North Korean confirmed on Friday that it was working to restart its plutonium-producing reactor.

'Verge of collapse'

The UN's International Atomic Energy Agency (IAEA) said earlier this week that Pyongyang ousted UN monitors, after asking them to remove seals and surveillance equipment from its Yongbyon reactor.

Pak Kil-yon, the North Korean deputy foreign minister, told the UN General Assembly on Saturday there was still a possibility of keeping the six-party talks on track.

"[North Korea] will continue to make every sincere effort toward the denuclearization of the whole Korean peninsula, but will not be indifferent to an attempt to offend our dignity and self-respect, and violate its sovereignty," he said.

The six-nation talks which involve China as host, the two Koreas, the US, Japan and Russia - began in 2003 with the aim of persuading North Korea to abandon its nuclear programmes.