Washington: North Korea has "at least one other" uranium enrichment site than that disclosed last month to US experts, US state department spokesman Philip Crowley said on Tuesday.
"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, it certainly reflects work being done at at least one other site," Crowley told reporters.
The statement from the US state department comes after reports surfaced in South Korea that North Korea is digging a tunnel at its known nuclear test site which would make it ready to conduct a third nuclear test by March, a move calculated to strengthen its hand as it seeks international talks.
A South Korean newspaper said on Wednesday that the amount of earth removed from the tunnel at the site in Punggye township in a northeastern region indicates the tunnel is about 500 meters (550 yards) deep, half the distance needed for a nuclear test.
North Korea is also speeding up work on new construction at its main Yongbyon nuclear complex, where it revealed a previously unknown uranium enrichment facility last month, the newspaper quoted intelligence sources as saying.
"North Korea is digging the ground pretty hard when it`s cold enough to freeze the ground at its two major nuclear facilities," an intelligence official was quoted as saying.
"At this rate, (the Punggye tunnel) will reach (the) 1 km that is needed for a nuclear test by March to May," a separate intelligence source was quoted as saying by the newspaper.
South Korea`s foreign ministry declined to comment immediately.
Analysts say North Korea`s unveiling of a modern uranium enrichment facility and preparations for another nuclear test are likely to be ploys to pull regional powers back to the negotiating table, where the impoverished state has in the past secured economic aid and diplomatic attention.
The country showed a uranium enrichment facility at the Yongbyon site to a U.S. expert in November.
North Korea conducted nuclear tests at the Punggye site in 2006 and 2009, when detonations in tunnels were detected by U.S. and South Korean monitoring. The U.N. Security Council condemned last year`s test and imposed tough sanctions aimed at banning North Korea`s arms trade and cutting off funding for such programs.
Ailing leader Kim Jong-il`s plan to transfer power to his son Jong-un is also creating domestic political pressure, as he tries to build legitimacy for the untested and previously unknown successor with military grandstanding, analysts say.
South Korea`s foreign minister said on Tuesday he suspected there were more facilities in addition to Yongbyon where the North was enriching uranium.