"Generally speaking a considerable degree of technology is necessary to miniaturize an atomic bomb" for a missile, Japan's defense chief Fumio Kyuma said.

"At this moment I have not received factual information that North Korea has technology that advanced to do so," he said.

Japan feels the most threatened by North Korea, which fired a long-range Taepodong-1 missile over its main island in 1998. The two countries have never established diplomatic relations.

North Korea apparently had a failed test launch in July of a new version of the Taepodong missile, which fell into the Sea of Japan (East Sea).

But amid a long deadlock in international diplomacy to disarm North Korea, experts warned that the communist regime could develop nuclear warheads within years.

"Even if North Korea may have failed with its nuclear test it has clearly shown its intention to continue developing nuclear warheads. Worst-case scenarios are no longer simple imagination," said Hideshi Takesada, military analyst at the National Institute for Defense Studies.

North Korea needs around two to three years to downsize its atomic bombs to 800 kilograms (1,760 pounds), which can be loaded onto its Nodong missiles, estimated another military analyst, Tadasu Kumagai.

But he said if North Korea made the decision, it could simply send a plane to Japan carrying an atom bomb -- although it would almost certainly be shot down.

"This is only a question of reducing the size of its atomic bombs. If it could, it would simply carry its atomic bombs on a plane and drop them here," Kumagai said.

North Korea said it carried out its test to provide a deterrent to US hostility and pledged it would never be the first to use nuclear weapons.

Japan and its allies have said they cannot confirm yet that North Korea has successfully entered the nuclear club.

South Korean experts estimate the explosion was the equivalent of 550 tonnes of TNT -- a very small amount for a first nuclear test.

The US bomb that destroyed Hiroshima in 1945, killing more than 140,000 people in the world's first nuclear attack, was comparable to 12,500 tonnes of TNT.

Japanese Prime Minister Shinzo Abe has pledged Wednesday to go ahead with sanctions regardless of whether North Korea's test was successful.

Abe promised to step up cooperation with the United States on missile defense. The US military stationed its first Patriot surface-to-air missiles in Japan after North Korea's missile tests in July.

The United States is treaty-bound to protect Japan, which was forced to become officially pacifist after its defeat in World War II.

With more confrontation expected, Japan's main options will remain its alliance with the United States and making use of its improving ties with China, which is the North's main ally.

"Japan should borrow the United States' military strength and should cooperate more with China so it can use its leadership in persuading North Korea," Kumagai said.

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