North Korea may have the bomb, but it hasn't perfected ways to put one onto a missile that could strike faraway enemies like the United States.
This is why Pyongyang's announcement that it will launch a satellite on a long-range rocket next month is drawing so much attention: Washington says North Korea uses these launches as cover for testing missile systems for nuclear weapons that could target Alaska and beyond.
Although North Korea isn't on the official agenda of next week's Nuclear Security Summit in the South Korean capital, here's a look at why the launch will be a major point of discussion when President Barack Obama and other world leaders gather in Seoul:
North Korea has spent decades trying to perfect a multistage, long-range rocket.
Next month's launch _ set to happen around the April 15 centennial of the birth of founder Kim Il Sung _ would be the fourth of its kind since 1998, when Pyongyang sent a long-range rocket hurtling over Japan.
A 2006 test was considered a failure, but North Korea grabbed attention shortly after with its first nuclear test blast. The U.N. Security Council later banned North Korea from any further nuclear or ballistic missile testing.
North Korea's third launch, in 2009, was considered a partial success, with two of the three stages pushing the rocket over the Pacific. The third stage failed, and, despite North Korea's claims of success, no satellite was put into orbit, the U.S. North American Aerospace Defense Command said.
That test was condemned by the U.N. Security Council. Pyongyang protested that it was testing satellite technology for peaceful purposes. It subsequently abandoned six-nation nuclear disarmament talks and, weeks later, carried out a second nuclear test.
The next year saw violence blamed on North Korea that killed 50 South Koreans, including an attack on a warship and the North's shelling of a front-line island.
Experts and governments will scrutinize next month's launch of what the North's state media call an Unha-3 rocket, presumably the next version of the Unha-2 rocket used in the 2009 test.
Unha-2 represented a significant advancement over previous rockets, according to an analysis written by missile experts David Wright and Theodore Postol. It was roughly 100 feet (30 meters) long and may have been designed around Soviet missile components, the writers said.
Next month's rocket is set to fire from a new site on the North's west coast, according to GeoEye and Google Earth satellite imagery posted by Tim Brown, an analyst for GlobalSecurity.org. The Tongchang-ri site is about 35 miles (56 kilometers) from the Chinese border city of Dandong, across the Yalu River from North Korea.
Positioned only 45 miles (70 kilometers) from the North's main Yongbyon nuclear complex, it has better roads and facilities, and allows a southerly flight path that keeps the rocket from flying over other countries, according to Wright, technical researcher at the Union of Concerned Scientists.
The new rocket will probably have better boosters and engines _ and might even succeed in putting a satellite into space if it contains one, said Sohn Young-hwan, a South Korean rocket scientist who heads the privately funded Institute of Technology and Management Analysis in Seoul.
North Korea may have loaded the rocket's third stage with more fuel to increase capability, Wright said by email, part of improvements that ``would translate to greater range if that technology was used to build a long-range ballistic missile.''
North Korea says the launch is meant to contribute to ``international trust and cooperation in the field of space scientific researches.''
But because ballistic missiles and rockets in satellite launches ``share the same bodies, engines, launch sites and other development processes, they are intricately linked,'' said Mark Fitzpatrick, an analyst at the International Institute for Strategic Studies.
So far, Pyongyang can only deliver a nuclear bomb ``by boat, by van or by airplane, not by missile,'' said Hecker, the nuclear scientist.
After half a century of persistence, North Korea is thought to have a fairly small nuclear arsenal.
While it has enough plutonium for about four to eight ``simple'' bombs similar to what the U.S. dropped on Nagasaki in 1945, Hecker estimates, it doesn't yet appear to have the ability to make bombs small enough to mount on a missile.
Miniaturized warheads would require more nuclear tests, and Hecker warns that if North Korea breaks its nuclear test moratorium, ``it will almost certainly be a test of a miniaturized design.''
Governments and experts are worried that a new rocket launch will spur a chain of events that will mirror 2009, resulting in a breakdown of diplomacy, another nuclear test and soaring tensions, threats and bloodshed.
The United States has warned the launch would jeopardize a diplomatic deal settled last month that would ship U.S. food aid to the impoverished North in exchange for a moratorium on missile and nuclear tests, as well as a suspension of nuclear work at Yongbyon.
U.S. officials will be pushing China to pressure its ally Pyongyang, and U.S. President Barack Obama is expected to raise the issue during a key meeting with Chinese President Hu Jintao on the nuclear summit's sidelines. There could also be meetings among the U.S. and its Asian allies, Japan and South Korea.
Hecker says a launch ``makes a mockery'' of the U.S.-North Korea nuclear deal.
``You use the same technology in long-range rockets that you do in long-range missiles,'' he said. ``The only difference is what you put on top.'' (AP)