Getting a nuclear device to explode is one breakthrough. Getting it to explode where you want it to, say experts, is another problem entirely. Now that North Korea appears to have joined the world's nuclear club, researchers say Pyongyang's next likely step is to develop a means of delivering the weapons by mounting them on missiles.
North Korea has invested heavily in missile technology for many years.
Kim Tae-woo is a senior analyst with Seoul's Korea Institute for Defense Analyses. He says two types of North Korean missile, the medium-range Rodong and the longer-range Taepodong lines, eventually may be capable of carrying a nuclear warhead.
Kim says a nuclear device would need to weigh one ton or less to be placed successfully on a missile. To fit on a Rodong missile, he says it should weigh no more than 500 kilograms.
North Korea tested at least seven missiles in July, including one long-range Taepodong-2 theoretically capable of reaching parts of the United States. That missile apparently failed within minutes of leaving the ground.
This week, Pyongyang said it had exploded a nuclear device. While international experts are not yet certain if there was a successful nuclear blast, it is clear, they say, that the isolated country is moving forward in its efforts to build a nuclear bomb.
Pyongyang says it needs nuclear weapons to prevent an attack by the United States. Washington has said repeatedly it has no intention of attacking North Korea, and calls on the country to comply with its past pledges to not build nuclear weapons.
Most analysts agree any nuclear weapon the North has developed is rudimentary, and bulky. Daniel Pinkston is a Korea specialist and East Asia Director at the Center for Nonproliferation Studies in Monterey, California. He says the dynamics of mounting a weapon on a missile are complex.
"You have to think about...integrating that [weapon's] design with the missile," he said. "You can't just stick anything on top of it, right? It's just like putting a big flatbed truck and weld it on top of the missile, it's not going to fly."
To have a credible long-range missile threat, experts say North Korea will need to factor in variables such as the heat and stress of leaving and re-entering the atmosphere.
Robert Hewson, who was an aerospace consultant before becoming an editor at Jane's Defense Weekly, points out that Pyongyang will also need a reliable way to aim a missile.
"It's not enough simply to point it in the right direction and hope it lands," he said. "So not only will they have to have all of the missile technology, the metals and material technology, the weapon technology - which is asking a lot - but they also need to have a guidance system."
Hewson say North Korea will probably need to develop its nuclear missile capabilities on its own - because international efforts to limit the spread of such weapons have made it extremely difficult to buy the technology or find engineers with such highly specialized skills.
Even if North Korea is a long way from being able to threaten the United States with a nuclear missile, experts warn Pyongyang's ability to intimidate its neighbors should be taken seriously. The country has demonstrated its missiles can hit South Korea and Japan, both long-time U.S. allies.
The other concern about North Korea's nuclear program, weapons experts say, is the chance that Pyongyang will try to sell its technology to other countries or terrorist groups. The United States is pushing for international cooperation to make sure does not happen.