The four UN inspectors had flown into North Korea unsure if they would be allowed to visit the Yongbyon reactor, which produces the raw material for bomb-making plutonium.

But the Japanese news agency Kyodo on Wednesday quoted Olli Heinonen, who is leading the International Atomic Energy Agency (IAEA) delegation, as saying the inspectors would travel to Yongbyon on Thursday.

When asked in Pyongyang about his discussions with North Korean officials thus far, Heinonen declined to comment beyond saying: "I think we had a good meeting."

The five-megawatt Yongbyon reactor, located 95 kilometres (60 miles) north of Pyongyang, was ostensibly built to generate electricity but is reportedly not connected to any power lines.

Instead, experts say, it has produced enough plutonium over 20 years for possibly up to a dozen nuclear weapons.

The last time UN inspectors were in North Korea was in 2002, but they were kicked out in December that year at the start of a crisis that led to the regime's first ever nuclear weapons test last year.

Under a February accord, the North has now promised to shut down and seal the Yongbyon facility under UN supervision in return for badly-needed energy aid and diplomatic concessions.

South Korean Foreign Minister Song Min-Soon voiced his optimism that North Korea would honour its promise, speaking as he left for Washington to discuss ways to speed up disarmament.

"After the consultation is over, I think it (the Yongbyon reactor) will be shut down as early as possible," Song told reporters. "That's not an issue that would take a political decision."

Song said he would meet Thursday with US Secretary of State Condoleezza Rice to discuss "how to structure the measures that will follow the initial actions for the denuclearisation."

Under the terms of the February accord, the North must eventually abandon the Yongbyon reactor. It also agreed to declare all of its nuclear programmes, including an enriched uranium-based scheme which it has denied operating.

As well as diplomatic benefits, such as talks on restoring diplomatic ties with Washington, the regime would also receive emergency energy aid equivalent to one million tons of heavy fuel oil.

Meanwhile the head of a European Union delegation that visited North Korea this week said he believed Pyongyang was committed to disarmament.

"We had a real impression that they are willing immediately (to carry out) the shutdown," Hubert Pirker, who led a European parliament team on a four-day trip to Pyongyang that ended Tuesday, told journalists here.

US nuclear envoy Christopher Hill, who last week became the highest-ranking US official to visit North Korea since 2002, has predicted it will shut down Yongbyon within three weeks.

He said he hoped the facility could be "disabled" by the end of the year.

Although the accord was agreed in February, its implementation was held up because of a dispute over North Korean funds frozen at a Macau bank.

They were released and finally returned at the weekend to Pyongyang.

North Korea appeared to have test-fired a short-range missile into the Sea of Japan on Wednesday as part of a routine military exercise, the South Korean Yonhap news agency said.