|The Yongbyon atomic site in North Korea: DigitalGlobe|
North Korea has "at least one other" uranium enrichment site in addition to the one shown to US experts last month, the US State Department has said.
The country's enrichment programme "reflects work being done at at least one other site", a spokesman said.
A New York Times report quoted unnamed intelligence officials, saying North Korea was using "significantly more advanced" nuclear technology than Iran.
Enriched uranium can be used for nuclear fuel or made into weapons.
North Korea revealed what appeared to be a fully operational uranium enrichment plant at its Yongbyon atomic complex to US nuclear expert Siegfried Hecker on 12 November.
Officials in the US and South Korea have said in recent days the facility shown to the expert last month could not have been constructed so quickly if other secret sites or another completed uranium enrichment plant did not already exist in the country.
"We're very conscious of the fact that, in the recent revelations to American delegations, what they saw did not come out of thin air," US state department spokesman Philip Crowley said.
(Click here to read the full report on the BBC News website.)