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".

North Korea lays out tough pre-conditions for talks
Seoul (AFP) April 18, 2013 - North Korea offered talks Thursday with South Korea and the United States, but laid out pre-conditions that Seoul and Washington dismissed and analysts said would do little to reduce soaring tensions.

The demands laid out by the North's main military body included the withdrawal of UN sanctions and a permanent end to South Korea-US joint military drills.

The offer followed a month of increasingly hostile exchanges between Pyongyang, Seoul and Washington that have included threats of nuclear war and precision missile strikes.

The North's conditions were swiftly rejected by South Korea which, together with the United States, has made any talks conditional on the North putting its nuclear weapons programme on the table.

"North Korea's demands are totally incomprehensible. It's absurd," foreign ministry spokesman Cho Tai-Young told reporters.

US Secretary of State John Kerry also dismissed the North's conditions, which emerged after an offer from the diplomat during his weekend visit to the Korean peninsula for Pyongyang to return to the negotiating table.

"That's the first word of negotiation or thought of that we've heard from them since all of this has begun," Kerry told US lawmakers.

"So I'm prepared to look at that as... at least a beginning gambit -- not acceptable, obviously, and we have to go further."

Dialogue has become the new focus of the blistering rhetorical battle that has trapped the Korean peninsula in an escalating cycle of military tensions ever since the North carried out its third nuclear test in February.

South Korea's new president, Park Geun-Hye, has made tentative -- and conditional -- offers of talks, but the North's initial response was to swat them away as a "crafty trick".

Some analysts see the North's engagement in a debate over dialogue -- no matter how unrealistic the conditions -- as a welcome shift from the apocalyptic threats that have been pouring out of Pyongyang.

"It's an initial show of strength in a game of tug-of-war that at least shows a desire to have a dialogue down the line," said Yang Moo-Jin, a professor at the University of North Korean Studies in Seoul.

But others like Daniel Pinkston, a North Korea expert with the International Crisis Group, ruled out any softening of Pyongyang's position and said those hoping for dialogue were being wilfully naive.

The North, Pinkston argued, had bound itself to a course that could only end with its recognition as a nuclear power -- a status that is anathema to the United States and its allies.

"So what is there to even talk about?" Pinkston said.

"The North is committed. It's burned its bridges. Any reversal could only be made at immense domestic cost to the regime.

"And there is simply no way any US administration is going to sit down and confirm a change in the status quo with the North as a nuclear state," Pinkston said.

"We're still firmly on a collision course, and it's not going to end well," he added.

The first step demanded by the North's National Military Commission was the withdrawal of "cooked up" UN sanctions that were imposed after the nuclear test in February.

North Korea has repeatedly cited the sanctions as a prime trigger for the current crisis.

The other main bone of contention has been ongoing South Korea-US military drills, which have involved the deployment of nuclear-capable B-52s and B-2 stealth bombers.

Both countries must provide international guarantees that such "nuclear war drills" will never be repeated, the commission said.

"Dialogue and war games can never go together," it added.

President Park's dialogue overtures to the North received the backing of Kerry during his recent Northeast Asia tour and UN chief Ban Ki-moon urged Pyongyang to "seriously" consider Seoul's offer.

Both Park and Kerry stressed any talks would have to be predicated on signals from North Korea that it would "change its ways" and respect its international obligations, especially with its nuclear programme.

But in Thursday's statement, the North stressed it had no intention of bargaining away its nuclear weapons.

"Nothing is more foolish than pressurising (North Korea) to show its will for denuclearisation first," it said.

The North's statement made no mention of a possible medium-range missile test -- the expectation of which has kept South Korean and US forces on heightened alert for the past week.

Intelligence reports suggest the North has two Musudan missiles primed to fire from its east coast, and most observers had predicted a launch on or around April 15, the birthday of the North's late founder Kim Il-Sung.