UN nuclear inspectors verified the shut down of North Korea's reactor, confirming the most significant move to curb the North's atomic ambitions in years, but more remains to be done, the head of the IAEA said on Monday.

North Korea said over the weekend it had shut its Soviet-era Yongbyon nuclear reactor, which provides the secretive state with material for arms-grade plutonium, around the time it received the first shipment of 6,200 tonnes of oil provided by Seoul as part of an aid-for-disarmament deal.

'The reactor has been shut down,' International Atomic Energy Agency director Mohamed ElBaradei told reporters in Bangkok. 'We have verified the shutdown of the reactor.'

The next step will be to verify that North Korea has shut other key facilities at Yongbyon, located about 100 km (60 miles) north of Pyongyang -- which include a plant to make plutonium.

'It's a very important step that we are taking this week, but it's a long way to go,' ElBaradei told reporters in Bangkok. He has said it will take IAEA personnel, who arrived in North Korea on Saturday, about a month to install seals and monitoring equipment to make sure Pyongyang keeps the reactor closed.

South Korea sent a second batch of 7,500 tonnes of oil to energy-starved North Korea on Monday, a Unification Ministry official said.

A provision of 50,000 tonnes of oil from the South is part of a Feb. 13 deal reached among the two Koreas, the United States, Japan, Russia and China on first suspending the operation of the North's nuclear facilities and then disabling them.

The impoverished North will receive an additional 950,000 tonnes of oil, security assurances and be better able to conduct international trade if it completely scraps its nuclear arms programme -- considered one of Asia's biggest security threats.

'With complete denuclearisation, everything is going to be possible,' said Christopher Hill, the chief US envoy to the nuclear talks.

Hill, in Seoul prior to heading to Beijing, called on the North to disable Yongbyon and provide a complete inventory of its atomic arms programme.

'I certainly have to anticipate that there will be problems in the future because I never thought it would take until July to get this first step done,' Hill told reporters. The six-way talks are set to resume on Wednesday in Beijing to map out the next phase of ending the nuclear programme.

In 1994, North Korea froze the Yongbyon reactor in exchange for energy aid. In late 2002, the United States accused Pyongyang of violating that deal by having a secret programme to enrich uranium for weapons.

In December 2002, North Korea said it was restarting the Yongbyon reactor. It disabled IAEA surveillance devices and expelled their inspectors. - Reuters