Sung Kim, the head of the US State Department's Korea desk, arrived in Beijing following discussions linked to the six-party talks on ending North Korea's nuclear weapons programmes.

He did not speak to reporters waiting for him at Beijing airport, but China's official Xinhua news agency quoted him as saying in Pyongyang before departing that the trip had been "useful".

In Beijing, Kim met separately with Chinese and Russian diplomats to brief them on his talks in Pyongyang, a US embassy official in Beijing said.

He later departed for Seoul where he would brief South Korean officials, while delegation member Paul Haenle, a Chinese expert at the US National Security Council, would brief Japanese officials in Tokyo, she added.

The team's visit to Pyongyang came after North Korea agreed earlier this month to disable key facilities at its Yongbyon nuclear complex and declare all other nuclear programmes by the end of the year.

In exchange for these actions, China, South Korea, the United States, Japan and Russia would supply North Korea with energy and other aid and offer up diplomatic concessions to the isolated nation.

Another team of US experts was expected to return to Pyongyang this week to continue discussions, Washington announced previously.

On Tuesday in Sydney, top US nuclear negotiator Christopher Hill said North Korea would also have to hand over weapons-grade plutonium produced at the Yongbyon plant to make further progress on an aid-for-disarmament deal.

Hill, a US assistant secretary of state, said that the North possessed 50 kilogrammes (110 pounds) of plutonium that would become the focus of international talks on North Korea in the new year.

"The issue will be to get this 50kg," he said. "We need to get the North Koreans to agree to abandon this 50kg. That's going to be the toughest sell."

Hill said an agreement on the material -- which has never been publicly admitted by North Korea, but is seen as crucial to the dismantling of its programmes -- would lead to sweeping moves toward peace.

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