Satellite images of activity at North Korea's main nuclear complex support Pyongyang's claims it is constructing an experimental light water reactor in defiance of global pressure to denuclearise, a US think tank says.
South Korean foreign minister Kim Sung-hwan said that by building such a reactor, the North was violating UN resolutions banning the secretive state from any nuclear activity.
Analysts say the construction of the new reactor, along with reports of activity at a nuclear test site which have fuelled speculation of a third atomic test, could be used as leverage by the North at the negotiating table.
North Korea has said it wants to return to stalled aid-for-disarmament talks, but both Seoul and Washington have dismissed its pledges to denuclearise as insincere.
Even though it has exploded nuclear devices, North Korea has not shown it has a working nuclear bomb.
North Korea has tried to secure a light-water reactor for a number of years, claiming such a project would be for peaceful energy purposes. The type of reactor is considered relatively proliferation-resistant, meaning it is unlikely to be diverted for an arms programme.
Analysts are sceptical of North Korea's ability to build a light-water reactor indigenously, because it requires key components that only advanced nuclear states such as the United States can provide.
The Institute for Science and International Security reported that images taken on November 4 showed the frame of a large building under construction at the Yongbyong site, saying this backed reports by experts that the North was building a reactor.
Siegfried Hecker, a former chief of the Los Alomos National Laboratory, and former US nuclear envoy Jack Pritchard, have both visited the North this month and reported that Pyongyang was building a 25-30 megawatt reactor.
"Dr Hecker informed ISIS that the new construction seen in the satellite imagery is indeed the construction of the experimental light water reactor," the Washington-based ISIS said on its website.
South Korea's Kim rejected the North's claims that it has the right to peaceful use of atomic energy, saying it is a right only under the nuclear nonproliferation treaty (NPT). North Korea quit the pact in 2003, just months after the latest nuclear standoff began in late 2002.
North Korea froze its Yongbyon nuclear site under a 2005 deal with five regional powers in return for aid, but is reported to have restarted activities there recently as the six-way disarmament process remains stalled for two years.
A five-megawatt graphite-moderated reactor at Yongbyon has produced arms-grade plutonium that officials and experts believe the North used to build several nuclear bombs.
Hecker was quoted by Japan's Kyodo news agency as saying the light water reactor would take several years to complete.
Pritchard - who met with the North's top nuclear envoy Kim Kye-gwan and Ri Gun, the North's deputy negotiator for the stalled six-party talks - said he was told by an official that the North wanted to complete the reactor by 2012.