North Korea has made progress in its nuclear weapons program to a level that it can produce weapons-grade uranium to make a bomb on its own, South Korea's defense chief said Wednesday.

"We evaluate that North Korea can build a nuclear weapon using uranium," Defense Minister Kim Kwan-jin said during an interpellation session at the National Assembly, giving a rare assessment on the reclusive nation's nuclear program.

In regard to North Korea's 5-megawatt reactor that was reactivated in April, Kim said Seoul is closely monitoring activities at the Yongbyon complex where a uranium enrichment plant and a reactor are located.

"We consider (the North) is in the test phase, keeping a close eye on the full-scale operation (of the reactor)," Kim said.

Pyongyang aims to become a "nuclear weapon state" to take the initiative in the six-party nuclear freeze deal and consolidate power domestically, Kim said.

The North has conducted three nuclear tests of increasing power since 2006, most recently in February. It is not yet known whether the recent test involved a plutonium or a uranium-based device.

The communist state is believed to have a handful of crude plutonium-based bombs, and Seoul officials believe it is ready to conduct the fourth test any time at its test site in the northeastern tip.

Last week, South Korea's vice defense minister Baek Seung-joo said that Pyongyang is expected to acquire 6 kg of weapons-grade plutonium by the end of next year if the Yongbyon nuclear reactor continues to operate in the current phase.

Many experts estimate, however, that Pyongyang has not yet mastered the miniaturization technology needed to mount a warhead on an inter-continental missile capable of reaching the U.S. shore.

The development further complicates the long-stalled efforts to stop a nuclear program that Pyongyang has vowed to expand, despite international condemnation and sanctions already placed on the impoverished state.

Fuel for North Korea's plutonium bombs has been made in a reactor that is large and easily monitored. But uranium-based weapons are more difficult for outsiders to investigate because the centrifuges needed to enrich uranium for bombs can be easily hidden from satellites. (Yonhap News)