North Korea is incapable of carrying out most of its threats of recent weeks, a prominent US expert who has visited the country's nuclear facilities several times said Thursday.
"The bark is much greater than the bite," Siegfried Hecker from Stanford University, who revealed in 2010 the existence of a uranium enrichment facility in North Korea, said in Vienna.
"All of these things that they have threatened to do, most of them they cannot do," said Hecker, currently a visiting scientist at the Vienna Center for Disarmament and Non-Proliferation.
He played down the significance of North Korea's April 2 announcement that it would re-start its main Yongbyon nuclear complex, mothballed in 2007.
Currently, he said, North Korea has "not much" plutonium -- between 24 and 42 kilogrammes (53-93 pounds), enough for four to eight weapons.
It will take six months for Yongbyon to be operating again -- and around three years until it is producing plutonium, he said, at a rate of one bomb's worth annually.
"So the world does not have to be concerned that when they made that decision to re-start that reactor, that tomorrow they would have more bombs. They can't have more bombs tomorrow," Hecker said.
Plutonium is however not the only fissile material that can be used in a nuclear bomb, with highly enriched uranium another possibility.
It remains unclear whether North Korea used uranium in its third nuclear test in February, or plutonium as in the first two.
That test drew toughened UN sanctions and set in motion a cycle of escalating tensions that has seen Pyongyang threaten nuclear strikes against the United States and South Korea.
Hecker said he interpreted Pyongyang's announcement that it planned to "readjust" Yongbyon as meaning that it intends to reconfigure a uranium enrichment facility there to produce weapons-grade material.
"If you re-plumb -- and that's all you have to do, reconfigure the facility -- you can make highly enriched uranium. On the basis of a quick calculation you can make about 40 kilos of highly enriched uranium per year, which is one to two bombs' worth," he said.
He also said he believed North Korea has an additional secret uranium enrichment site, though not with a "huge capacity".
Even this though is not a "game changer", and North Korea is still some distance from having a nuclear warhead on a missile, Hecker said.
He said Pyongyang was "at least five years" away from having a missile capable of reaching the United States, and that even short-range missiles still required "some significant testing".
"But you still have worry about it. It is a serious situation," he added, however, saying that Pyongyang could carry out another nuclear test in the "next weeks to months".