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.
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.
Associated Press writers Sam Kim and Jean H. Lee in Seoul and AP researcher Zhao Liang in Beijing contributed to this report.
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