Siegfried Hecker, who was shown the North's modern uranium enrichment facility during a visit to the country in November last year, urged regional powers to stop Pyongyang from staging another nuclear test and more missile tests.
North Korea conducted two nuclear tests in 2006 and 2009 and is believed to have enough plutonium for at least a half-dozen nuclear bombs. But the Stanford University professor and other experts said North Korea has yet to master the miniaturization technology to mount a nuclear warhead on a missile.
"If North Korea conducts a third nuclear test, that will be very risky," Hecker told a forum in Seoul. "If another of the North's nuclear tests is successful, I believe that North Korea will succeed in the necessary miniaturization within a few years.
"It is critical at this point to bring Pyongyang back to the table to stop expanding its nuclear weapons programs," Hecker said. "The most urgent step is to stop it from conducting another nuclear test and more missile tests."
North Korea rolled out the road-mobile Musudan intermediate-range ballistic missile during a military parade in October last year,
Hecker said the missile is capable of carrying a nuclear warhead.
South Korean intelligence officials have warned that North Korea could carry out another nuclear test at any time, although there are no signs of preparations.
A more difficult and relatively less urgent issue is stopping North Korea from constructing large numbers of additional centrifuges, but the diplomatic objective for denuclearization on the Korean Peninsula should remain focused on the "immediate danger," said Hecker of Stanford University.
Siegfried Hecker speaks during a forum in Seoul on Dec. 14. (Yonhap)
A flurry of renewed diplomatic efforts have been underway since early this year to resume the long-stalled six-party talks on ending the North's nuclear weapons ambition.
The six-nation talks, involving the two Koreas, the U.S., China, Russia and Japan, have been dormant since April 2009, when the North quit the negotiating table and then conducted its second nuclear test a month later.
North Korea has called for an early resumption of the six-party talks without preconditions, but South Korea and the U.S. insist Pyongyang must first take concrete steps to show its sincerity, such as a monitored shutdown of its uranium enrichment plant.
On Nov. 30, North Korea announced that its low-enriched uranium production efforts are "progressing apace," raising tensions and casting clouds over efforts to revive the six-party talks,
Pyongyang claims the uranium enrichment program is for peaceful energy development, but outside experts believe it will give the country a new source of fission material to make atomic bombs, in addition to its widely known plutonium-based nuclear weapons program.