On Wednesday, May 20, North Korea announced it was able to produce an atomic warhead small enough to fit on a ballistic missile. State media in the communist country quoted a spokesman for the National Defense Commission as saying the country could defend itself with atomic weapons, a report by South Korea's Yonhap News Agency said. According to this report, the country "has reached the stage of ensuring the highest precision and intelligence and best accuracy of not only medium- and short-range rockets but long-range ones".

This would represent a big advancement in North Korea's nuclear and missile program. Reducing the size of a nuclear warhead to the point where it can fit on a long-range missile would be a major leap forward in offensive capability. But Pyongyang has claimed progress with miniaturization before - which later turned out to be false. The United States and South Korea directly expressed skepticism. "Our assessment of North Korea's nuclear capabilities has not changed. We do not think that they have that capacity" to miniaturize weapons, a National Security Council (NSC) spokesman, told AFP news agency.

This view is shared by nuclear scientist and former Director of the Los Alamos National Laboratory Siegfried Hecker, who talks to DW about why he believes the threat of a North Korean submarine armed with nuclear-tipped ballistic missiles is many years away.

DW: Earlier this month, North Korea announced it had launched a ballistic missile from a submarine. The National Defense Commission now claims to have perfected the technology required to make nuclear warheads fit a missile. How reliable is this information?

Siegfried Hecker: Not reliable and not believable, especially for a long-range missile. It is not even certain that the missile was launched from a submarine rather than a submersible platform.

Experts say the hardest part of delivering a nuclear strike is shrinking a warhead to fit a missile. If the information turns out to be true, how much of a breakthrough would that be for North Korea?

Nuclear weapons can also be delivered by truck, boat or plane, which allows for much larger nuclear weapons to be accommodated. What makes missiles attractive for nuclear delivery is that they can reach much greater distances in a very short time. Hence, they are much more threatening. In other words, a nuclear tipped missile is a much better deterrent.

North Korea has demonstrated that it can build a nuclear device. By its own admission, it has stated that the first two nuclear tests were not of a small, miniaturized design, but the third one was. It is likely that North Korea was able to make the third tested device smaller than the first two, but it is highly unlikely that it is small enough to fit on a long-range missile.

With its limited test experience, it is not possible for them to have confidence to mount such a device on a long-range missile. With their limited nuclear test experience it is also not clear that they can mount a nuclear warhead on medium or short-range missile with confidence.

Claims like this one have been made before and so there is skepticism among experts. How difficult is it to provide estimates regarding the state of affairs of the North Korean nuclear program?

It is very difficult to estimate the size and nature of the North Korean nuclear program. Since there is virtually no direct intelligence about these programs, one has to interpret what they claim and what one can see from satellite imagery.

If confirmed, this would allow the deployment of nuclear weapons far beyond the Korean Peninsula. How much of a threat would it pose to neighboring countries and the stability of the region?

The threat of a North Korean submarine armed with nuclear-tipped ballistic missiles is many years away. They continue to work to achieve such capabilities, but have not come close to date.

What kind of strategy do you believe could prevent further tensions and the risk of a military escalation?

I have advised for many years that we should focus on stopping the expansion of North Korea's nuclear arsenal first, and subsequently focus on denuclearization. It is what I call a strategy to halt, then roll back, and eventually eliminate the nuclear weapons program. The final step will take many years in my opinion.

Siegfried Hecker is a nuclear scientist and professor at the Department of Management Science and Engineering at Stanford University, in the United States. The internationally recognized expert in plutonium science, global threat reduction and nuclear security and former Director of the Los Alamos National Laboratory has traveled frequently to North Korea. In November 2010 he visited the nuclear complex in Yongbyon.