The nine experts had begun work on plutonium production facilities at the Yongbyon complex in North Korea, said Tom Casey, a State Department spokesman speaking in Washington.

The team led by Sung Kim, the head of the department's Korea desk, "had in fact arrived at Yongbyon and they are beginning their activities there in terms of starting with the first aspect of disablement of the facility," Casey told reporters.

"Yes, the process has started," he said. "Obviously it is going to be a process that is going to take some time."

The North, which staged its first nuclear test in October 2006, has agreed with five negotiating partners to declare and disable all its programmes by year-end in return for energy aid and major diplomatic benefits.

In July it took the first step by shutting down its reactor at Yongbyon. Disablement aims to make the reactor and other plants unusable for at least a year while talks on total denuclearisation continue.

The North will receive energy aid worth hundreds of millions of dollars in return for disablement.

If it goes on next year to dismantle the plants and give up its plutonium stockpile and nuclear weapons, it can expect normalised relations with Washington and a peace pact to replace the armistice which ended the 1950-1953 Korean War.

The aim of disablement is to avoid a re-run of what happened in 2002, when a 1994 denuclearisation pact with the United States fell apart.

Despite an eight-year shutdown the North quickly resumed production of plutonium and now has an estimated 45-65 kilogrammes (99-143 pounds) -- enough to build several bombs.

South Korea's Yonhap news agency, quoting diplomatic sources, said chief nuclear negotiators from the six nations -- the two Koreas, the United States, China, Japan and Russia -- recently agreed 11 disablement measures at the three major plants at Yongbyon.

It said these include the withdrawal of about 8,000 spent fuel rods from the five-megawatt reactor, the only one in operation in the country.

Yonhap said the removal of the rods, which weigh some 50 tonnes in total, was expected to take at least six weeks. The US team was expected to keep them in a cooling pond until a decision is made on how to dispose of them.

A six-nation pact reached in February also envisages the North's eventual removal from a US list of state sponsors of terrorism, and from the provisions of the Trading with the Enemy Act.

But US envoy Christopher Hill said Saturday that Pyongyang would first have to satisfy Washington that it was not engaged in any terrorism-related activities.

"They have to address the terrorism concerns that put them on the list in the first place," he said.

North Korea's quest for nuclear weapons began after the Korean War, when Washington stationed nuclear warheads in South Korea and Japan.

In the mid-1950s, Pyongyang signed a research agreement with Moscow under which hundreds of its scientists were trained in nuclear physics by the Soviets. The North later signed a similar cooperation agreement with China.

The US withdrew its nuclear weapons from the Korean peninsula in the early 1990s.

More recently, according to US sources, the North turned to Pakistan to develop a separate programme based on highly enriched uranium. Pyongyang has reportedly agreed to account for that programme as well.

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