North Korean soldiers keep watch on the DMZ between the two countries

North Korea has "good technical reasons" to carry out at least one more atomic test if it wants to develop a nuclear-armed missile, a prominent U.S. scientist who has often visited the reclusive Asian state said on Friday.

The North tested nuclear devices in 2006 and 2009, but still has not shown it has a working nuclear bomb, reports Reuters.

Proliferation experts have said the country has enough fissile material for up to 10 nuclear weapons, but they don't believe it can miniaturise one to place on a missile.

Stanford University's Siegfried Hecker, who late last year was shown a previously undetected uranium enrichment facility in North Korea, said he believed the isolated state knew how to build a "relatively simple, rudimentary plutonium bomb".

But, "I don't believe they could have confidence on the basis of those tests to make one small enough to mount on a missile," Hecker told a seminar for diplomats in Vienna.

"So if they want ... to have the confidence that they had one they can mount on a missile, if they want to convince the rest of the world, they would need at least one other nuclear test."

In October last year, a South Korean government source was quoted as saying that a U.S. satellite had detected increased activity at a North Korean nuclear weapons test site, suggesting it could be preparing for a third test.

But tensions have eased on the peninsula this year and a flurry of diplomacy has raised hopes for a resumption of regional talks on disabling the North's nuclear programme.

Hecker suggested a limited availability of fissile material in the North as well as the reaction of the outside world, including that of its biggest patron China, could make the North think twice about conducting any more tests.

"But in my opinion there is still a very good possibility that, under what they would consider the right political circumstances, they may do another nuclear test," he said.

"Certainly, technically I would think that that is what they would want to do."

North Korea often uses Hecker, a former director of the U.S. Los Alamos National Laboratory where the atomic bomb was developed, as its preferred foreign scientist to demonstrate its nuclear capabilities. He has visited seven times since 2004.

In November 2010, it showed him an enrichment facility at its Yongbyon nuclear complex, stoking regional tension as the advance could give it a second route to making nuclear arms.

The isolated state says it is only pursuing uranium enrichment for peaceful energy purposes. As for its plutonium programme, Pyongyang says it was cornered into pursuing nuclear weapons because of the United States' nuclear threat.

"My conclusion of the North Korean nuclear programme is, yes, they have a bomb, but not much of a nuclear arsenal," Hecker said, adding there was a low threat the North would actually use one. He suggested the proliferation risk Pyongyang posed was a more immediate concern. Much of the North's income in the past has been generated through arms sales.

Last year, a U.N. report suggested the North may have supplied Syria, Iran and Myanmar with banned atomic technology.

"The world knows that North Korea did a lot of exporting of missile technology. Now it appears that North Korea has also gone into the nuclear export business," Hecker said. "That to me is the most immediate threat, needs to be stopped now."