The US has said a report that North Korea has built a new nuclear facility is further evidence of Pyongyang's "belligerent behaviour".
The top US military officer, Adm Mike Mullen, said North Korea was "continuing on a path which is destabilising for the region".
A US scientist said he had seen "more than 1,000 centrifuges" for enriching uranium on a visit to North Korea.
Enriched uranium can be used for nuclear fuel or made into weapons.
"From my perspective, it's North Korea continuing on a path which is destabilising for the region," Adm Mullen, the head of the Joint Chiefs of Staff, told CNN.
"It confirms or validates the concern we've had for years about their enriching uranium, which they've denied routinely," he said.
In September last year, after having denied enriching uranium, North Korea said it was in the final stage of uranium enrichment, and further warned that it was continuing to reprocess and weaponise plutonium.
Adm Mullen said the latest report of the North's nuclear activity should be seen in the light of the March sinking of a South Korean warship, which Seoul and Washington blamed on Pyongyang.
The sinking of the Cheonan in a suspected torpedo attack left 46 South Korean sailors dead and inflamed tensions on the Korean peninsula.
"All of this is consistent with belligerent behaviour, the kind of instability creation in a part of the world that is very dangerous," Adm Mullen said.
His remarks followed the publication of a report by US nuclear scientist Siegfried Hecker on his trip last week to North Korea's Yongbyon nuclear complex, which is about 100km (60 miles) north of the capital Pyongyang.
He said he had been shown an experimental light-water nuclear reactor that was still under construction and a new facility that contained "more than 1,000 centrifuges" that the North Koreans told him was processing low-enriched uranium for fuel for the new reactor.
The North Koreans told him the facility contained 2,000 centrifuges.
He said the facility seemed designed primarily for civilian nuclear power but could be easily converted to further process uranium to weapons grade.
The plant was modern and clean, unlike all the other Yongbyon facilities he had seen, and he was stunned at how sophisticated it was, the Stanford University scientist said.
He also said the North Koreans told him the new plant was "constructed and operated strictly with indigenous resources and talent".
When international weapons inspectors were expelled from North Korea in 2009, the plant did not exist, officials say.
The North is believed to have weaponised enough plutonium for at least six atomic bombs but is not known to have a uranium-based weapons programme.
The report came as Stephen Bosworth, a senior US state department official responsible for North Korea, was travelling to Asia to try to revive six-party talks on Pyongyang's nuclear programme.
Beginning with a stop in the South Korean capital Seoul, Mr Bosworth will then travel to Tokyo and Beijing.
North Korea has nuclear and missile programmes and conducted underground atomic tests in 2006 and 2009.
The speed with which the country is pressing ahead with its nuclear programme will deepen suspicions that it is receiving help from abroad in circumventing United Nations sanctions, correspondents say.
The North has reportedly expressed a conditional willingness to return to the stalled talks and some analysts suggest it may have revealed these new uranium enrichment facilities in a bid to strengthen its negotiating hand.