UN inspectors on Thursday headed to a nuclear reactor at the centre of North Korea's weapons programme for the first time in nearly five years, a day after the communist state conducted new missile tests.
Olli Heinonen, head of the four-strong UN team visiting North Korea since Tuesday, told China's Xinhua news agency in Pyongyang that "we are going to see the facilities and continue our discussion in more detail."
The delegation is expected to return to Pyongyang on Friday afternoon, Xinhua quoted Heinonen as saying.
"A working-level delegation from the International Atomic Energy Agency (IAEA) left for Yongbyon to visit the nuclear facilities," Xinhua said.
The access conforms to a landmark February deal, under which the North pledged to shut down the five-megawatt reactor under UN supervision in return for badly-needed energy aid and diplomatic concessions.
The 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.
UN inspectors were kicked out in December, 2002, at the start of a crisis that led to the regime's nuclear weapons test last year.
The new IAEA team has been tasked with arranging the shutdown of North Korea's nuclear reactor at Yongbyon.
The visit to Yongbyon came after the US confirmed that North Korea had test-fired "several missiles" off its east coast on Wednesday.
"The United States is deeply troubled that North Korea has decided to launch these missiles during a delicate time in the six-party talks," US National Security Council spokesman Gordon Johndroe said in a statement.
He said the missile launches violated a UN Security Council resolution banning North Korea from engaging in all ballistic missile activities, urging Pyongyang to focus on nuclear disarmament.
But Pentagon officials soon played downed the firing of "short-range missiles" as a routine military exercise, not intended to be provocative.
Under the February deal, 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 rewards, the Pyongyang regime would receive emergency energy aid equivalent to one million tons of heavy fuel oil and diplomatic benefits, such as talks on restoring diplomatic ties with Washington.
US chief nuclear negotiator 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 also hoped the facility could be "disabled" by the end of the year.
Officials in Seoul and Washington expect the six-party talks aimed at terminating Pyongyang's nuclear programme to resume in July. The talks involve host China, the US, the two Koreas, Japan and Russia.
South Korean Foreign Minister Song Min-Soon is in Washington this week for talks with US Secretary of State Condoleezza Rice to discuss measures that will follow the shutdown of Yongbyon for the denuclearisation.