"We want this disabling to take place by December 31st. So we have to look at our ideas for disabling against the actual facility," the diplomat, Christopher Hill, told reporters. "And we thought 'the sooner, the better.'"

"There are many different ways you can disable a nuclear facility: You can drill a hole in the side of a reactor, you can fill it with cement, you can do various things, but it helps if you have a site survey, and have a look at the reactor first so that's the concept here," he explained.

The experts, who will report back to the next meeting of the six-party talks on the issue, will work with North Korea officials on issues like "which facilities they'll have a look at," said Hill.

"It's an ambitious phase, we have a lot of work to do and I think it's a sign of the seriousness of purpose that all parties, including the North Koreans, bring to bear on this issue," he said.

Hill, the assistant US secretary of state for East Asian and Pacific affairs, said the experts were going to North Korea at the invitation of the government of leader Kim Jong-il.

They will "conduct a survey of nuclear facilities to be disabled as part of the six-party process aimed at denuclearisation of the Korean peninsula," he said. "I don't think it will be the only such trip."

The six-country talks began in 2003 and group China, Japan, Russia, the United States, North and South Korea.

North Korea has already shut down its nuclear reactor at Yongbyon under an agreement reached on February 13.

Under the deal, North Korea agreed to make a full declaration of all its nuclear programmes and to disable them in return for aid, security and diplomatic guarantees, notably normalisation of ties with Washington.