SEOUL, South Korea -- The U.S. special envoy for North Korea said Monday that Pyongyang's claim of a new uranium enrichment facility is provocative and disappointing but not a crisis or a surprise. Washington, he vowed, will keep working closely with its regional partners in response.

Stephen Bosworth's comments, following a meeting with South Korean Foreign Minister Kim Sung-hwan, came as the United States and the North's neighbors scrambled to deal with Pyongyang's revelation to a visiting American nuclear scientist of a highly sophisticated, modern enrichment operation that had what the North says are 2,000 recently completed centrifuges.

"This is obviously a disappointing announcement. It is also another in a series of provocative moves" by North Korea, Bosworth said. "That being said, this is not a crisis. We are not surprised by this. We have been watching and analyzing the (North's) aspirations to produce enriched uranium for some time."

Kim also played down the facility, telling reporters: "It's nothing new."

Top U.S. military officials, however, warned that it could speed up the North's ability to make and deliver viable nuclear weapons. South Korea's defense minister told lawmakers Monday that Seoul will discuss the possibility of having the U.S. bring tactical nuclear weapons back into the country.

U.S. Defense Secretary Robert Gates said the facility could enable North Korea to build "a number" of nuclear devices beyond the handful it is presumed to have already assembled. Gates was speaking in Bolivia, where he is attending a regional defense conference.

The American scientist, Siegfried Hecker, posted a report over the weekend saying that during a recent trip to the North's main Yongbyon atomic complex, he was taken to a small, industrial-scale uranium enrichment facility.

Hecker, a former director of the U.S. Los Alamos Nuclear Laboratory who is regularly given glimpses of the North's secretive nuclear program, said the North Korean program had been built in secret and with remarkable speed.

It wasn't immediately clear why the North chose to reveal the previously hidden facility. It could be a ploy to win concessions in nuclear talks or an attempt to bolster leader Kim Jong Il's apparent heir. The North could also be serious about producing nuclear electricity.

Regardless, it provides a new set of worries for the Obama administration, which has shunned direct negotiations with North Korea following its nuclear and missile tests last year and in the wake of an international finding that a North Korean torpedo sank a South Korean warship in March, killing 46 sailors.

The United States has been working with China, Japan, Russia and South Korea since 2003 to persuade North Korea to abandon its nuclear programs through a framework known as the six-party talks. Bosworth, who plans to visit China on Tuesday, traveled later Monday to Tokyo for discussions with Japanese Foreign Minister Seiji Maehara.

"If what North Korea is claiming is really true, it's an extremely grave problem," Maehara said at the outset of his meeting with Bosworth. "We must respond calmly, and will step up our cooperation, particularly among Japan, the U.S. and South Korea."

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 told Hecker it began construction on the centrifuge facility in April 2009 and finished only a few days before the scientist's Nov. 12 visit.

The facility appeared to be primarily for civilian nuclear power, not for North Korea's atomic arsenal, Hecker said. But, he said, it "could be readily converted to produce highly enriched uranium bomb fuel."

Uranium enrichment would give the North a second way to make nuclear bombs, in addition to its known plutonium-based program. At low levels, uranium can be used in power reactors, but at higher levels it can be used in nuclear weapons.

Asked about the possibility of resuming the stalled six-nation nuclear disarmament talks with the North, Bosworth said U.S. officials "do not at all rule out the possibility of further engagement with North Korea." But, he added, "I do not believe in engagement just for the sake of engagement or talking just for the sake of talking."

Bosworth said the uranium revelation does not constitute a failure of U.S. policy toward the country's nuclear programs and that Washington will work closely with "our allies and partners" going forward.

"This is a very difficult problem that we have been struggling to deal with for almost 20 years," Bosworth said. "They are a difficult interlocutor ... but we're not throwing our policy away."

New satellite images show construction under way at Yongbyon, which, combined with reports from Hecker and another American expert who recently traveled to the atomic complex, appear to show that the North is going forward with its stated plans to build a light-water nuclear power reactor.

Light-water reactors are ostensibly for civilian energy purposes, but such a power plant would give the North a reason to enrich uranium. While light-water reactors are considered less prone to misuse than heavy-water reactors, once the process of uranium enrichment is mastered, it is relatively easy to enrich further to weapons-grade levels.

Experts say the North has yielded enough weaponized plutonium for at least a half dozen atomic bombs.

Hecker said the North Koreans emphasized during his trip that the centrifuge facility was operating; although he couldn't verify that statement, he said "it was not inconsistent with what we saw."