United Nations inspectors have confirmed the shutdown of North Korea's Yongbyon nuclear reactor, the head of the UN's nuclear watchdog has said.

Mohamed Elbaradei praised what he said was "good co-operation" from North Korea, and welcomed the shutdown as a "step in the right direction".

"Our inspectors are there. They verified the shutting down of the reactor yesterday," he told reporters during a visit to Bangkok.

The chief US nuclear negotiator with North Korea also welcomed what he called "a good start", but said much work remained to be done.

"We took a long time to get these first steps and we have really a lot work to do now, but I think we're off to a good start," Christopher Hill, an assistant US secretary of state, told reporters in the South Korean capital, Seoul.

Hill said the disablement of the North's nuclear facilities could be completed by the end of the year, and he expected a complete declaration of all North Korea's atomic programmes within months.

But he added there were still a number of steps left before they could reach complete denuclearisation, and he expected that the long delays in shutting down the Yongbyon plant meant there could still be problems ahead.

Hill's comments came as South Korea sent a second shipment of oil to the North, in line with six-nation disarmament-for-aid deal signed in Beijing in February.

The tanker carrying 7,500 tonnes of oil left Ulsan in the South for the North Korean port of Nampo.

The first shipment of oil arrived in the North on Saturday, shortly after which North Korean officials announced they had shut down the Yongbyon reactor.

The two shipments are part of an initial 50,000 tonnes of oil that was pledged to the North in return for closing the reactor.

Pyongyang is to receive a further 950,000 tonnes once it has shut down its nuclear weapons programme.

The shutdown of the Yongbyon reactor, located about 100km north of Pyongyang, is the first step taken by North Korea to scale back its nuclear programme since it expelled UN nuclear inspectors in 2002.

On Saturday a 10-member team of inspectors from the International Atomic Energy Agency returned to North Korea on Pyongyang's invitation.

Under the six-nation deal, aside from energy aid the North will receive security assurances and the dropping of international sanctions among other benefits if it completely and verifiably scraps its nuclear arms programme.

"With complete denuclearisation, everything is going to be possible," Hill told reporters in Seoul.

He and other negotiators in the six-party talks are expected to travel to Beijing this week to discuss plans for pushing the disarmament process forward.

PHOTO CAPTION

A DigitalGlobe satellite image shows a nuclear facility in Yongbyon, North Korea September 29, 2004. (Reuters)