North Korea says it has thousands of centrifuges operating at a previously undetected uranium enrichment facility it revealed earlier this month.
The North Koreans say the plant is for civil nuclear power. It is not clear whether the centrifuges could be used to produce material for weapons.
A US scientist has seen the facility, but could not confirm it was working.
The claim comes as tensions remain high, after the North shelled a South Korean island a week ago.
Pyongyang says the shelling, which killed four people, was in response to Seoul's military drill near Yeonpyeong island.
It has described a four-day US-South Korean military exercise now under way in the Yellow Sea, near the disputed border between the two Koreas, as a provocation.
The aircraft carrier, the USS George Washington, and four other US navy vessels are being joined by South Korean destroyers, patrol vessels, frigates, support ships and anti-submarine aircraft. BBC News
