An American nuclear scientist says he was shown a vast new nuclear facility when he visited North Korea last week.
The scientist, Siegfried Hecker, told the New York Times he had been stunned by the plant's sophistication.
Professor Hecker was shown what he describes as hundreds and hundreds of centrifuges for enriching uranium, which can be used to make nuclear weapons.
American officials say that in April 2009 when international weapons inspectors were expelled from North Korea this plant did not exist.
The BBC reports that the speed with which the country is pressing ahead with its nuclear programme will fuel suspicions that it is receiving help from abroad to circumvent United Nations sanctions.