North Korea is building an experimental light-water reactor at its Yongbyon nuclear facility, Kyodo news reported on Saturday, citing Siegfried Hecker, former chief of the Los Alomos National Laboratory.
Hecker told reporters in Beijing that he was informed of the construction by North Korea and the output of the reactor was on a scale of 25 to 30 megawatts, according to the report.
Hecker recently visited North Korea, the report said. He also said that the construction has just begun and it would take several years to complete it.
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.
A 1994 deal between North Korea and the United States was intended to provide the destitute state with two 1,000-megawatt light-water reactors built by an international consortium, but the agreement fell apart in 2006 after the North was accused of pursuing a nuclear 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.
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.