Kang Myong-do, 36, told a news conference in the South Korean capital Seoul that North Korea was genuinely intent on developing a nuclear weapons capability. It was not just playing on Western suspicions about its nuclear plants to obtain diplomatic concessions.
He said the head of operations at the North's nuclear plant at Yongbyon told him in October 1993 that Pyongyang had five nuclear bombs.
"I understand North Korea had completed developing five nuclear warheads by the end of last year," he said.
"It sees nuclear development as the only means to maintain Kim Jong-il's regime," he said, referring to the son of North Korea's late "Great Leader" Kim Il-sung who has apparently smoothly succeeded his father as leader.
Seoul's Agency for National Security Planning said Kang had sought asylum in the South in May via a third nation.
Prime Minister Kang Song-san was ranked third in the North's hierarchy, after Kim Jong-il and Defence Minister O Jin-u, when it announced the funeral committee for Kim Il-sung, who died on July 8. The prime minister is also a cousin of Kim Il-sung.
Kang said Pyongyang would reveal its nuclear program to the world after making about 10 warheads by the end of the year "to secure an advantageous position in its talks with the United States and Japan.
"North Korea believes the United States would not be able to attack it then," he said.
Earlier this year, the U.S. Central Intelligence Agency said there was a better than even chance the North had extracted enough plutonium from Yongbyon in 1989 to build at least one nuclear weapon. CIA director James Woolsey said in June that failure to halt the program could mean it could have enough plutonium for five more bombs later this year.
But Michael Breen, a Seoul-based analyst, doubted if Kang would have had access to reliable information about the programme despite his connections.
"I think the facts about the nuclear program are known by very, very few people in North Korea," he said.
He said Seoul may have persuaded Kang to make his statement because it was not eager to hold a North-South presidential summit postponed.
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