Intelligence authorities in South Korea and the U.S. believe North Korea has already succeeded in manufacturing small nuclear warheads, which can be mounted on a 1,300 km-range missile, a member of a private think thank claimed Tuesday.

North Korea maintains Rodong missile bases in North Pyongan Province, Jakang Province and Yangkang Province.

In a telephone interview with the Chosun Ilbo, Daniel Pinkston, a senior analyst for the International Crisis Group's Northeast Asian office in Seoul, quoted an anonymous intelligence officer as saying North Korea is storing nuclear warheads in underground storage facilities, with their plutonium and trigger devices separated. He claimed it would take about two or three days to assemble and mount them on missiles.

The South Korean government had until recently speculated that North Korea has six to eight nuclear weapons, but had remained doubtful whether the North had managed to miniaturize the technology sufficiently to make missile warheads.

In January, North Korea told visiting American academics that out of 38.5 kg of plutonium it reported to the six-party talks, it had already turned the disposable amount of 30.8 kg into weapons. At that time, North Korean General Ri Chan-bok implied that weaponized plutonium had been used to make warheads.