Hwang Jihwan, a North Korea expert at the University of Seoul, said the North "is keeping tension and crisis alive to raise stakes ahead of possible future talks with the United States."
"North Korea is asking the world, `What are you going to do about this?'" he said.
The unidentified North Korean atomic spokesman said the measure is meant to resolve the country's acute electricity shortage but is also for "bolstering up the nuclear armed force both in quality and quantity," according to a statement carried by the official Korean Central News Agency.
The statement suggests the North will do more to produce highly enriched uranium, which like plutonium can be used to make nuclear weapons. Uranium worries outsiders because the technology needed to make highly enriched uranium bombs is much easier to hide than huge plutonium facilities. North Korea previously insisted that its uraniumenrichment was for electricity - meaning low enriched uranium.
Kim Jin Moo, a North Korea expert at the Korea Institute for Defense Analyses in South Korea, said that by announcing it is "readjusting" all nuclear facilities, including a uranium enrichment plant, North Korea "is blackmailing the international community by suggesting that it will now produce weapons-grade, highly enriched uranium."
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U.N. Secretary-General Ban Ki-moon said Tuesday that North Korea appears to be "on a collision course with the international community."
Speaking in Andorra, the former South Korean foreign minister said the crisis has gone too far and international negotiations are urgently needed.
China, Pyongyang's only major economic and diplomatic supporter, expressed unusual disappointment with Pyongyang. "We noticed North Korea's statement, which we think is regrettable," said Chinese Foreign Ministry spokesman Hong Lei. Seoul also called it "highly regrettable."
The North's plutonium reactor generates spent fuel rods laced with plutonium and is the core of Nyongbyon. It was disabled under a 2007 deal made at now-dormant aid-for-disarmament negotiations involving the North, the U.S., South Korea, Japan, China and Russia.
In 2008, North Korea destroyed the cooling tower at Nyongbyon in a show of commitment, but the deal later stalled after North Korea balked at allowing intensive international fact-checking of its past nuclear activities. Pyongyang pulled out of the talks after international condemnation of its long-range rocket test in April 2009.
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North Korea "is making it clear that its nuclear arms program is the essence of its national security and that it's not negotiable," said Sohn Yong-woo, a professor at the Graduate School of National Defense Strategy of Hannam University in South Korea.
Pyongyang conducted its third nuclear test in February, prompting a new round of U.N. sanctions that have infuriated its leaders. North Korea has since declared that the armistice ending the Korean War in 1953 is void, shut down key military phone and fax hotlines with Seoul, threatened to launch nuclear and rocket strikes on the U.S. mainland and its allies and, most recently, declared at a high-level government assembly that making nuclear arms and a stronger economy are the nation's top priorities.
The Korean Peninsula is technically is a state of war because a truce, not a peace treaty, ended the Korean War. The United States stations 28,500 troops in South Korea as a deterrent to North Korea.
Washington has said it takes the threats seriously, though White House press secretary Jay Carney said Monday the U.S. has not detected any military mobilization or repositioning of forces from Pyongyang.
The North's rising rhetoric has been met by a display of U.S. military strength, including flights of nuclear-capable bombers and stealth jets at annual South Korean-U.S. military drills that the allies call routine but that Pyongyang claims are invasion preparations.
South Koreans are familiar with provocations from the North, but its rhetoric over the last few weeks has raised worries.
"This is a serious concern for me," said Heo Jeong-ja, 70, a cleaning lady in Seoul. "The country has to stay calm, but North Korea threatens us every day."
Earlier Tuesday, a senior South Korean official told foreign journalists that there had been no sign of large-scale military movement in North Korea, though South Korea remains alert to the possibility of a provocation. The official spoke on condition of anonymity because he was not authorized to speak publicly to the media.
North Korea added its 5-megawatt plutonium reactor to its nuclear complex at Nyongbyon in 1986, and Pyongyang is believed to have exploded plutonium devices in its first two nuclear tests, in 2006 and 2009.
There had long been claims by the U.S. and others that Pyongyang was also pursuing a secret uranium program. In 2010, the North unveiled to visiting Americans a uranium enrichment program at Nyongbyon.
Analysts say they don't believe North Korea currently has mastered the miniaturization technology needed to build a warhead that can be mounted on a missile, and the extent of its uranium enrichment efforts is also unclear.
Scientist and nuclear expert Siegfried Hecker, one of the Americans on the 2010 visit to Nyongbyon, has estimated that Pyongyang has 24 to 42 kilograms of plutonium _ enough for perhaps four to eight rudimentary bombs similar to the plutonium weapon used on Nagasaki in World War II.
It's not known whether the North's latest atomic test, in February, used highly enriched uranium or plutonium stockpiles. South Korea and other countries have so far failed to detect radioactive elements that may have leaked from the test and which could determine what kind of device was used.