By James Pearson

SEOUL (Reuters) - North Korea tested its most powerful nuclear device on Friday, but the more worrying aspect for its rivals was Pyongyang's claim that it had advanced its ability to make a nuclear weapon, by miniaturizing and mounting a warhead on a missile.

Its KCNA news agency said the test had used a nuclear warhead that had been "standardized to be able to be mounted on strategic ballistic rockets".

"The standardization of the nuclear warhead will enable the DPRK to produce at will and as many as it wants a variety of smaller, lighter and diversified nuclear warheads of higher strike power," KCNA said, referring to the North's formal name, the Democratic People's Republic of Korea.

There is little scientific evidence to verify that North Korea has perfected the science of creating a nuclear bomb small enough to fit on a ballistic missile and withstand the physics of atmospheric re-entry.

But it may be getting closer.

In April, a South Korean official said the North had accomplished miniaturization of a warhead, although the Pentagon said it did not necessarily share that assessment.

In March, North Korean leader Kim Jong Un met nuclear scientists working on miniaturization. He was photographed at the visit looking at a small, ball-like device KCNA said was a miniaturized nuclear warhead. He was also photographed inspecting a missile nosecone.

"They called the A-bomb they displayed 'small-type' or 'miniaturized', but it might be better to say 'compact'," said Joshua Pollack, editor of the Nonproliferation Review. "It looked bigger than what we would call miniaturized, but they wanted to show us that it would fit into an ICBM (inter-continental ballistic missile) nosecone."

"If it fits that nosecone, it ought to fit on a Scud or Rodong as well," said Pollack. The Rodong is a North Korean medium-range missile tested earlier this week that can fire a 1 ton (1,100 lb) warhead up to 2,000 km (1,250 miles).

That would bring all of South Korea and Japan within range.

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After Friday's test, however, experts were more focused on the intensity of the blast.

Earthquakes triggered by North Korean nuclear tests have gradually increased in magnitude since Pyongyang's first test in 2006, indicating the isolated country is steadily improving the destructive power of its nuclear technology.

After Friday's test, South Korean monitors said the blast caused a 5.0 magnitude earthquake. The U.S. Geological Survey (USGS) measured a magnitude of 5.3.

In terms of energy, the USGS estimation translates to a nuclear blast potentially 2.8 times stronger than the estimation given by the South, according to a USGS tool used to calculate earthquake strength.

LITTLE BOY & FAT MAN

After its fourth test, in January, North Korea boasted it had tested a hydrogen bomb, although the yield of that explosion indicated it was more likely to have been a boosted atomic bomb.

A boosted bomb is not as powerful as a standard H-bomb, but can efficiently increase the explosive power of an A-bomb by using small amounts of radioactive hydrogen isotopes.

Boosting means North Korean scientists can get more explosive yield out of a smaller amount of fissile material, stretching their small stockpile of plutonium further, getting more bang for their buck.

"This means they might have more bombs than a simple plutonium count would leave you to believe," said Jeffrey Lewis of the California-based Middlebury Institute of International Studies.

In terms of explosive yield, the USGS measurement points to a North Korean bomb more powerful than the "Little Boy" bomb dropped on Hiroshima, and almost as powerful as the "Fat Man" bomb dropped on Nagasaki.

The South Korean measurement would indicate a yield slightly smaller than the Little Boy at 10 kilotonnes, although calculating yield is difficult and subject to the unknown variables in terrain and geology of the test site.

North Korea's first nuclear test in 2006 created a magnitude 4.3 earthquake, according to USGS data. Friday's bomb-induced 5.3 earthquake was a magnitude and therefore ten times bigger, indicating a significantly more powerful nuclear blast.

(Editing by Raju Gopalakrishnan)