Most experts are keeping an open mind on North Korea's nuclear weapons programme because the country is so tightly controlled and they have too little information to work on.

Many believe that Pyongyang has several nuclear bombs but that they are so big they cannot effectively be loaded into planes, let alone fired by missiles.

The technology to build the bomb is one thing, but in order to use them effectively, particularly on ballistic missiles, North Korea would have to acquire the tricky skill of miniaturising a nuclear warhead.

When North Korea said in February that it possessed nuclear weapons, confirming long-held suspicions in the United States, South Korea's National Intelligence Service said Pyongyang probably lacked the technology to fire them on a rocket.

"North Korea might have developed one or two conventional nuclear bombs, but if it did, it may not have the technology to launch them on a missile," the NIS report said.

"We believe North Korea has not acquired enough technology to miniaturize nuclear bombs which must weigh less than 500 kilograms to be mounted on a missile."

North Korea has a well-advanced missile programme and among Washington's greatest fears is that Pyongyang could breech the technical threshold of marrying its ballistic missile development with its nuclear weapons drive.

In April, the head of the US Defense Intelligence Agency, Vice Admiral Lowell Jacoby, told a Senate committee hearing in Washington that US intelligence agencies believed North Korea had mastered the technology for arming its missiles with nuclear warheads, though he did not say Pyongyang had actually done so.

The Pentagon later took a step back from Jacoby's assessment, saying it was "theoretical in nature" but US President George W. Bush said it was safer to err on the side of overstatement when dealing with North Korean capabilities.

"There is concern about his capacity to deliver," he said. "We don't know if he can or not, but I think it's best when dealing with a tyrant like Kim Jong Il to assume that he can."

Abdul Qadeer Khan, the disgraced father of Pakistan's nuclear bomb who is now under house arrest, has admitted playing an important role in North Korea's atomic development.

Clandestine cooperation between Khan and the North Koreans since the 1990s has reportedly included the provision of warhead designs to North Korea.

Khan has also claimed that during one of his many visit to North Korea he saw a missile carrying a nuclear warhead.

Many leading experts think North Korea may have developed crude nuclear weapons similar to the devices dropped by the United States on Hiroshima and Nagasaki in 1945.

The Nagasaki "Fat Man" bomb weighed more than four tonnes and was overloaded with chemical explosives used to trigger the plutonium blast. North Korean nuclear scientists have been working feverishly to refine their version.

Throughout the 1980s and 1990s, they carried out more the 100 tests of high-explosive triggers that would help them in their weaponization efforts.

"We don't know for certain, but North Korean scientists may have been able to weaponize a Rodong missile," said Kang.

North Korea's medium range Rodong can travel up to 1,300 kilometers, meaning it is capable of hitting targets in most areas of Japan.

Pyongyang in 1998 test-fired a Taepodong-1 missile with a range of up to 2,500 kilometers that overflew Japan and is said to be developing the Taepodong-2 with a range of 6,700 kilometers.

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