* North Korea could conduct new nuclear test soon - Hecker
* More needed to make weapon small enough for missile
* Pyongyang's "bark much greater than its bite" (Adds quotes on North Korea's capabilities, paras 3, 5, 6, 17)
By Fredrik Dahl
VIENNA, April 18 (Reuters) - North Korea would need to carry out at least one more nuclear 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 could conduct its fourth such explosion in weeks or months.
"I still don't think they have enough nuclear-testing experience," Hecker told a seminar of the Vienna Center for Disarmament and Non-Proliferation.
The North has threatened nuclear attacks on the United States, South Korea and Japan after new United Nations sanctions were imposed in response to its latest nuclear arms test in February, the third since 2006.
Hecker, who is believed to have been the last Westerner to visit North Korea's Yongbyon nuclear complex, said the situation was serious but Pyongyang did not have the capability to carry out most of its threats.
"The bark is much greater than the bite but you still have to worry about it," he said.
Pyongyang's nuclear arms still are probably primitive and it would likely need several more tests to be able to make one small enough for a missile and have "sufficient confidence that you can put a nuclear weapon on a warhead", Hecker said
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.
"That would be the simplest delivery mechanism. However, it is very difficult to pull that off," he added. "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 SPY AGENCY REPORT
Signalling 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 the 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.
"They (North Korea) are very determined people," Hecker said.
"They can probably develop an ICBM (Intercontinental Ballistic Missile), they can probably miniaturise nuclear weapons. But they need lots of missile tests and they need more nuclear tests."
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. (Editing by Michael Roddy)