North Korea would need to carry out at least one more atomic test in order to develop a nuclear missile, a prominent U.S. scientist who has often visited the isolated Asian state said on Thursday.
Stanford University's Siegfried Hecker, who in 2010 was shown a previously undetected uranium enrichment facility in North Korea, said he believed it might be able to conduct its fourth such explosion soon, in weeks or months.
The North has threatened nuclear attacks on the United States, South Korea and Japan after new U.N. sanctions were imposed in response to its latest nuclear arms test in February.
"My view is that they need at least one more nuclear test and most likely several more nuclear tests,'' said Hecker, who is believed to have been the last Westerner to visit North Korea's Yongbyon nuclear complex.
"It is primarily in order to be able to miniaturize [a bomb to fit on a missile] and have sufficient confidence that you can put a nuclear weapon on a warhead,'' he told reporters during a visit to Vienna.
The most important and serious short-range threat could instead be delivery of a nuclear bomb by other means than a missile, for example on a boat or even in a car or van, Hecker said.
"That would be the simplest delivery mechanism. However, it is very difficult to pull that off,'' he said. "In the shorter term, most likely a boat would be the most serious threat.''
Missile launches and nuclear tests by North Korea are both banned under U.N. Security Council resolutions.
North Korea deems its nuclear arms a "treasured sword'' and has vowed never to give them up.
Pentagon Agency Report
Signaling a possible end to weeks of hostility on the Korean peninsula, however, North Korea on Thursday offered the United States and South Korea a list of conditions for talks, including the lifting of U.N. sanctions.
The United States has offered talks, but on the pre-condition that they lead to North Korea abandoning its nuclear weapons ambitions.
Hecker made clear he did not agree with a Pentagon spy agency report that triggered alarm last week that North Korea might be able to deliver a nuclear-tipped missile at a time of heightened tensions in Asia over Pyongyang's threats of war.
The evaluation from the Defense Intelligence Agency (DIA), produced in March and revealed at a congressional hearing, concluded that North Korea likely has nuclear bombs that could be delivered by missiles.
The Obama administration has played down the DIA report.
Based on the number of nuclear and missile tests the North had carried out, "I just don't believe they have that capability yet and certainly they couldn't possibly have the confidence yet to put one of those on a missile,'' Hecker said.
He added: "You have to test, both a nuclear test and then the combination of the nuclear warhead, or dummy warhead, with the missiles.''
North Korea occasionally lets experts like Hecker into the country, most likely to persuade them that it is not bluffing over its nuclear capabilities, U.N. diplomats and officials say.