South Korean intelligence authorities declined to comment on the report in the Monthly Chosun, which said that the defector, a man believed to be in his 70s using the alias Kim Il-Do, defected to the South in May.

"North Korea has built a one-tonne nuclear bomb by using four kilogrammes of plutonium," he was quoted as telling the National Intelligence Service (NIS), South Korea's spy agency.

The North was now seeking to miniaturize the bomb to make it more reliable as a weapon, he reportedly said. The man claimed he had been in the North's parliament and had worked for the Marine Industrial Institute.

"We interview escapees from the North to verify their IDs and check their motives but we don't comment on any other specifics about them," an NIS spokesman said.

But a spokesman for North Korea's foreign ministry dismissed the report, according to Pyongyang's official Korean Central News Agency (KCNA).

"The fiction is a sheer fabrication as there is no such institution in the north as the marine industrial institute to which he allegedly belonged. The statement allegedly made by him was full of lies," the spokesman said.

The United States says North Korea may possess one or two crude nuclear bombs and may have reprocessed enough plutonium for half-a-dozen more, from spent fuel rods at its Yongbyon nuclear complex.

More than 4,000 North Koreans have defected to the South since 1953. The number has shot up in the past four years, with more than 1,000 having reached South Korea last year.

Many defectors have arrived in South Korea, mostly via China, to escape famine caused by natural disasters and failed economic policies in their Stalinist homeland.

Professor Kim Young-Soo of Sogang University, an expert on defectors from the North, said he could not confirm the existence of the defector.

"However, the allegation that North Korea has produced a one-tonne nuclear bomb sounds plausible," he said.

A new round of six-nation talks aimed at bringing about the dismantling of North Korea's nuclear programs is due to be held in Beijing next week.

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