Based on what appears to be a successful test of an atomic bomb more powerful than the one North Korea detonated in 2006, former U.S. government and independent analysts say the North's technical skills are improving slightly.
Of greater concern, according to national security adviser James L. Jones, is the possibility that North Korea could sell or share its nuclear technology with others. He would not say whether the U.S. intelligence community judged the test to be a significant step forward.
"The fact that they have this kind of technology and are obviously willing to export it is very troubling," Jones told reporters Tuesday.
The U.S. government remains officially mum on technical details about the underground blast that took place Monday in North Korea. It could be days or weeks before radioactive gases from the underground test are detected and analyzed by U.S. and allied intelligence.
North Korea appears to be marginally closer to having both a nuclear warhead and the means of delivering it to the United States or U.S. allies in the Pacific. In April, Pyongyang launched a long-range missile in what represented a modest improvement over earlier missile tests. The government reported launching a series of short-range missiles on Tuesday.
The nuclear test proves that North Korea's basic warhead design works, said Charles Vick, a missile expert with Global Security.org. The next challenge is reducing its weight by about half, and then integrating the warhead onto a missile.
North Korea has said it has begun harvesting plutonium from spent fuel rods at its main nuclear plant to build up its atomic arsenal. It is thought to have enough weaponized plutonium to make more than a half-dozen atomic bombs.
In its official announcement of the test, North Korea said the explosion was larger than its 2006 test. Publicly available information supports that.
The U.S. Geological Survey recorded a seismic event — equivalent to an earthquake with a roughly 4.6 on the Richter scale — near the test site. The 2006 test registered roughly 4.1, according to the Comprehensive Nuclear Test Ban Treaty Organization, an arms control group.
Martin Kalinowski, a Hamburg University professor, says that corresponds to an explosive yield of about 3 to 8 kilotons of TNT, with a yield most likely of 4 kilotons.
That would be considerably higher than the half-kiloton yield in 2006. One kiloton is the equivalent of exploding 1,000 tons of TNT. For comparison, the U.S. nuclear bombs detonated in Japan during World War II ranged between 15 to 21 kilotons.
Jeffrey Lewis, director of nuclear strategy initiative at the New America Foundation, a think tank in Washington, cautioned that "it's really hard to know what the yield actually is." He said it might be as low as 1.5 kilotons.
Lewis described the range as "somewhere between what you wouldn't want dropped on your neighborhood, but it's nowhere near what was dropped on Hiroshima."
Discussions about yield and capability are somewhat beside the point, he added. The fact that North Korea has a weapon, and its intentions for it are unclear, are the true concerns.
"What's frightening is that they have a nuclear weapon, the uncertainty that comes from having a nuclear weapon, not the capability itself," said Lewis.
North Korea's nuclear test forced the Pentagon to scrap much of its planning for a meeting Saturday in Singapore among Defense Secretary Robert Gates and his Japanese and South Korean counterparts.
"Undoubtedly, the developments in North Korea over the weekend will be a focus of that conversation," Pentagon spokesman Geoff Morrell told reporters. He said it was believed to be the first discussion among the three nations' defense chiefs.
Associated Press writers Robert Burns and Lara Jakes contributed to this report.