The UN nuclear watchdog should revise its view on the amounts of fissile material North Korea needs to build a nuclear weapon as Pyongyang could have mastered miniaturization technology, a former agency official advises.

Olli Heinonen, a former deputy director general of the International Atomic Energy Agency (IAEA), made the suggestion in an article to the website of the Foundation for Defense of Democracies, emphasizing it's important to "not underestimate the skills and know-how of North Korean scientists and technicians."

"The possibility that North Korea has likely advanced in the miniaturization of its warhead requires modifications to the IAEA's verification objectives. The IAEA currently considers 8 kilograms of plutonium as a significant quantity of concern -- namely the minimum quantity of nuclear material required for the manufacture of nuclear explosive devices," he said.

"Instead, 2 to 4 kilograms of plutonium is more in line for North Korea, given its nuclear strides and success in miniaturization," he said. "Similarly, the goal quantity for highly enriched uranium has to be adjusted down from the current level of 25 kilograms of isotope U-235 to 15 kilograms."

His estimate means that the North could produce two or three plutonium bombs a year from the 5-megawatt reactor at its Yongbyon nuclear complex, instead of one as previously estimated. The North has also a second way of building atomic bombs through uranium enrichment.

Siegfried Hecker, a Stanford University professor known for having a first-hand look at North Korea's uranium-enrichment facility during his 2010 visit, said earlier this month that the North is believed to be capable of producing seven nuclear warheads worth of fissile material a year -- one bomb worth of plutonium and six bombs worth of highly enriched uranium.

Heinonen said the US must determine whether the North has mastered miniaturization of a nuclear warhead.

"This has never been independently verified, and some experts have cast doubt on the claims. Nevertheless, after a series of tests and evidence of increased inventories of plutonium and perhaps highly enriched uranium, North Korea appears closer to its stated goal," he said.

He also said the North's claims of mastering nuclear warhead standardization should not be "summarily dismissed."

"Numerous UN Security Council resolutions, presidential, and other statements have failed to bring the North Koreans back to the negotiating table. The regime is already heavily sanctioned, and China is unlikely to substantially increase pressure on North Korea," Heinonen said.

"New approaches are required to halt further development on North Korea's nuclear capabilities, and to eventually reach an agreement on the dismantlement of those capabilities," he said. (Yonhap)