It is the first time in nearly five years that International Atomic Energy Agency inspectors will have had access to the Yongbyon reactor, a source for potentially bomb-making material.

In a related development, Seoul announced it would hold talks for the first time on supplying impoverished North Korea with an initial tranche of energy aid promised in a February disarmament agreement.

Under that deal, the communist country vowed to shut down the five-megawatt Yongbyon site under UN supervision in exchange for badly-needed energy and diplomatic concessions.

Olli Heinonen, head of the four-strong UN team that has been in North Korea since Tuesday, was quoted as saying in Pyongyang that "we are going to see the facilities and continue our discussion in more detail."

The team is expected to return to Pyongyang Friday, China's Xinhua agency quoted him as saying as he left for Yongbyon.

Officials from the two Koreas will meet Friday and Saturday on providing 50,000 tons of fuel oil at the North Korean border town of Kaesong, the South's unification ministry said.

Yongbyon, 95km north of Pyongyang, was ostensibly built to generate electricity but it is reportedly not connected to any power lines.

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

UN inspectors were kicked out of North Korea in December 2002 at the start of an international standoff that led to the regime testing a nuclear weapon for the first time last year.

Under the February deal, the North must eventually abandon the reactor for good and come clean on all of its nuclear programmes, including an enriched uranium-based scheme, which it has denied operating.

If that is judged successful, Pyongyang would eventually receive energy aid equivalent to one million tons of heavy fuel oil.

US envoy Christopher Hill, who last week became the most senior US official to visit the North since 2002, has predicted it will shut down Yongbyon within three weeks. He hopes the plant could be "disabled" by the end of the year.

Meanwhile Japanese Prime Minister Shinzo Abe demanded a tough international response to the North's latest missile tests on Wednesday.

"We must seek a severe response in the international community," Abe said. "North Korea must respond to the concerns of the international community.

Pentagon officials played down the launches as a routine military exercise that was not intended to be provocative, while South Korea's defence ministry said the launches were aimed at testing new ballistic missiles.

Japanese Foreign Minister Taro Aso and his Chinese counterpart Yang Jiechi spoke by telephone Thursday and were upbeat about the progress in the nuclear issue, Xinhua reported.

Japan and China are two of the six nations involved in talks on the North's nuclear drive. The others are the two Koreas, Russia and the United States.

Officials in Seoul and Washington expect full six-party talks to resume in July.

Russia's envoy on North Korea Alexander Losyukov indicated his country would support four-nation talks excluding Tokyo and Moscow to help push forward the peace process.

"If there is a need to convene such a group of countries... I don't think we must be jealous or have something against that if the whole affair is directed at achieving the same goal that we have in common," he said in Tokyo.

South Korean president's top security advisor Baek Jong-Chun, who is on a trip to China, said Seoul and Beijing agreed to step up efforts to disable the North's nuclear programme.