U.S. Defense Secretary Robert Gates said he doesn't believe the facility is part of a peaceful nuclear energy program.
"I don't credit that at all," Gates said in Bolivia, where he is attending a regional defense conference.
The facility appears to be a uranium enrichment facility, Gates said, and it could enable North Korea to build "a number" of nuclear devices beyond the handful it is presumed to have already assembled.
Adm. Mike Mullen, chairman of the U.S. Joint Chiefs of Staff, called North Korea "a very dangerous country."
"I've been worried about North Korea and its potential nuclear capability for a long time," Mullen said on ABC's "This Week."
"This certainly gives that potential real life, very visible life that we all ought to be very, very focused on."
North Korea has tested two relatively crude plutonium devices. The new facility uses uranium, which can be used both for civilian power production and to make bombs.
North Korea has not yet demonstrated the capability to refine bomb-making skills to the point that the devices could be attached to long-range missiles. That ability would be needed if the North ever intended to launch a catastrophic attack far beyond its borders, including on the U.S.
The United States and other countries are also worried that North Korea could sell the abilities it now has to terrorists or other nations that might use them for small-scale "dirty bomb" attacks.
Gates warned that North Korea is developing new long-range missiles, and possibly a new mobile intercontinental ballistic missile.
"All of these programs are of great concern to every nation," Gates said.
North Korea recently showed off the new facility to a visiting American nuclear expert. The scientist said he was told the small industrial-scale uranium enrichment facility is producing low-enriched uranium for a new reactor.
Robert Carlin, a North Korea expert at Stanford University, also toured the site and said it was "way beyond anything anybody had imagined."
In an interview broadcast Sunday on ABC, Carlin said, "It was such a surprise, and it showed me that the policy that we've been following seems to be at a dead end."
Gates said the revelation Saturday confirms long-standing suspicions that North Korea wanted to develop a second path to nuclear capability.
Mullen, the top U.S. military officer, said such activities would violate U.N. Security Council resolutions and agreements by North Korea over its nuclear program.
"From my perspective, it's North Korea continuing on a path which is destabilizing for the region. It confirms or validates the concern we've had for years about their enriching uranium, which they've denied routinely," Mullen said. "They are a country that routinely we are unable to believe that they would do what they say."
Noting the sinking of the South Korean warship Cheonan in March, which killed 46 sailors and has been blamed on North Korea, Mullen said on CNN's "State of the Union" that "all of this is consistent with belligerent behavior, the kind of instability-creation in a part of the world that is very dangerous."
The United States wants North Korea to resume international disarmament talks with Japan, China, Russia, South Korea and the U.S. Those on-again, off-again negotiations yielded a 2005 promise from North Korea to give up its nuclear program, and it did dismantle some facilities before talks foundered.
North Korea has demanded one-on-one negotiations with the United States, which Washington refuses to hold in a formal setting. U.S. diplomats have met with North Koreans on the sidelines of the six-nation talks.
Gates said it is too soon to say how news of the new facility might affect diplomatic strategy.
Sen. John Kerry, chairman of the Senate Foreign Relations Committee, pointed to China as a key. The Massachusetts Democrat said Beijing is an influential ally and trading partner of the North and is "well-positioned to enhance the international community's enforcement activities."