North Korea, in a dramatic announcement last week that it will test its first atom bomb, said "safety will be thoroughly secured" -- a possible reference to an underground experiment.

A series of reports based on diplomats and intelligence data have said that North Korea has drilled a hole in a mountain near the northern town of Chiktong for the nuclear test.

If conducted properly, a test either inside a mountain or underground would contain radiation.

But the technical prowess of the impoverished state, which apparently failed in July to test-fire a long-range missile, remains largely a mystery.

Japan's conservative Sankei Shimbun newspaper said Saturday that North Korea was believed to have dug into a mountain for a test. But, citing Japanese government-backed research, it warned a surface test could bring disaster.

If North Korea were to conduct its test above ground, radioactive fallout -- known as the "ash of death" -- would fly to the northern half of Japan, parts of South Korea and Russia and a vast eastern area of China within 54 hours, the daily said.

Such a scenario would revive memories of the Chernobyl disaster, in which radioactive dust spread across a large swathe of Europe after explosions at a nuclear reactor in Ukraine.

Up to 9,000 people are expected to die from illness due to Chernobyl, according to UN figures disputed by environmentalists.

Even if North Korea intends to contain radiation, experts questioned whether it has the level of technology to conduct a nuclear test completely safely.

"Their nuclear test is feared to cause radioactive contamination to some extent, considering the lack of money and technology on a radiation shield," said Koh Yu-Hwan, a professor of North Korean studies at Seoul-based Dongkuk University.

"Due to geological reasons, it's hard for North Korea to dig a deep vertical underground tunnel for a nuclear test," Koh said.

"We worry that the test will contaminate underground water."

Narushige Michishita, a North Korea specialist at Japan's National Institute for Defense Studies, said if Pyongyang's nuclear test ends up polluting the region, ironically enough it would be its allies that would be most affected.

"I don't think radiation would fly to Japan," Michishita said. "At the farthest, it would spread to China, Russia and South Korea -- the North's buddies."

China, North Korea's largest provider of aid, and Russia have warned against further isolating the communist state. Both Beijing and Moscow wield veto power at the UN Security Council, where Japan and the United States have threatened a harsh response if Pyongyang goes ahead with a test.

South Korea also has adopted a so-called "sunshine policy" of trying to engage its estranged communist neighbor.

North Korea -- which US President George W. Bush branded part of an "axis of evil" -- said it would test a nuclear bomb due to hostility by the US.

Pyongyang last year agreed in principle to give up its nuclear program in exchange for aid and security guarantees. But it walked out of six-nation talks in November to protest US sanctions on a North Korean-linked bank.

Japan, the only country to have suffered nuclear attack, is the most concerned about North Korea, which fired a missile over its main island in 1998.

Baku Nishio, a senior member of Japan's non-profit Citizen's Nuclear Information Center, feared long-term consequences from a North Korean test.

"Radiation could start leaking out of the ground afterwards," Nishio said. "We don't even know what scale of test the North Koreans want to conduct."

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