With UN inspectors due back in North Korea at the weekend, the head of the UN's atomic watchdog agency said he expected the shutdown of the Yongbyon reactor to start early next week and to go smoothly.

Nine months after the hardline communist state shocked the world with its first atomic test, hopes were rising for the imminent closure of the reactor, which produces bomb-making raw material.

China announced that six-nation nuclear disarmament talks would resume next Wednesday after a four-month break.

North Korea has said it will consider shutting the Soviet-era Yongbyon facility as soon as it receives the first oil shipment, which was due to arrive on Saturday.

"I expect that operation to move smoothly," International Atomic Energy Agency (IAEA) chief Mohamed ElBaradei said of the initial shutdown, describing it as "not a complicated process."

"This is going to start early next week," he told a press conference in Seoul as his inspectors prepared to leave Vienna for Pyongyang to verify the move. "I am quite optimistic. This is a good step in the right direction."

But he cautioned that full denuclearisation is "going to be a long process. We should not delude ourselves. This has been a problem for over 15 years, the Korean nuclear issue, and it will take time to have a comprehensive solution."

The No. 9 Hanchang set sail from the southeastern port of Ulsan with 6,200 tons of heavy fuel oil aboard, carrying a banner reading "First shipment of 50,000 tons of heavy fuel oil for North Korea."

It was expected to reach North Korea's Sonbong port around Saturday after a 38-hour voyage.

IAEA staff were also expected to arrive Saturday for their first inspection mission since a previous nuclear accord collapsed in 2002, after US claims that the North was running a secret highly enriched uranium (HEU) project in addition to its plutonium operation.

That alleged programme, which the North denies operating, will be one of many contentious issues to be tackled in the second phase of a February six-nation disarmament deal.

The two Koreas, the United States, China, Russia and Japan have been negotiating since 2003. They will meet in Beijing on July 18 and 19, the Chinese foreign ministry said.

Under their February pact the energy-starved North will receive one million tons of fuel oil or equivalent aid, plus major diplomatic benefits and security guarantees, if it declares and dismantles all nuclear programmes.

Yongbyon's closure, to be rewarded with an initial 50,000 tons of oil from South Korea, is the first step.

ElBaradei said his inspectors in the second phase would have to ensure that all programmes, including any enriched uranium project, are declared.

"That is a process that will obviously take some time because we will also have to make sure that if they have nuclear weapons, as they say they do, these weapons should be dismantled."

Any nuclear settlement would be "good for the DPRK (North Korea), good for East Asia and good for the international community," he said.

North Korea says a denuclearised peninsula was the final wish of founding president Kim Il-Sung but that it needs the bomb to deter US aggression.

Diplomatic relations with the United States and a peace pact formally ending the 1950-53 war are on offer if it scraps all nuclear projects and weaponry.

But US ambassador Alexander Vershbow cautioned Wednesday that Washington would not settle for a partial solution which would leave North Korea "with even a small number of nuclear weapons."

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