North Korea has "very serious nuclear capabilities" and it is able to conduct another round of nuclear tests "any time," South Korea's nuclear envoy said Tuesday.

The communist country has restarted a nuclear reactor at its Yongbyon nuclear center, a provocative move that would provide Pyongyang with enough plutonium to build one atomic bomb a year, according to Seoul's spy agency.

Pyongyang conducted its third nuclear test in February.

"The assessment on North Korea's nuclear capabilities is very grave, and we see it has the ability to carry out another round of nuclear tests whenever it wants in technological terms," said Cho Tae-yong, South Korea's top nuclear envoy during a forum held in Seoul.

"There are signs that the five-megawatt graphite moderated reactor has been in operation recently, the North is expanding nuclear enrichment facilities, and construction is underway for a small-scale light-water reactor," Cho said.

The reactor in Yongbyon, located 90 kilometers north of Pyongyang, has been cited for producing plutonium for the North's nuclear weapons program before it was shut down under a deal brokered at the six-party talks six years ago.

The nation had blown up the reactor's cooling tower in 2008 to show its seriousness about suspending operations.

Pyongyang, however, announced in April when tensions spiked on the Korean Peninsula that it would restart work on the small reactor, which does not generate any energy, and use it to strengthen the country's nuclear deterrence.

Cho said that the North's unrelenting efforts to advance its nuclear capabilities are expected to make China deal with its communist neighbor "just as any other country."

As a way to induce North Korea to abandon its nuclear ambitions, Cho said the international community "has been strengthening its stick approach and pressuring (the North) now rather than the using the carrot," adding the multilateral talks "will go nowhere at this juncture."

The six-party talks, involving two Koreas, the U.S., China, Japan and Russia, were suspended in late 2008, and Seoul and Washington have urged Pyongyang to take concrete and meaningful steps toward denuclearization as a precondition to resume the forum.

"Even if the talks begin, we should not reduce pressure on the North, such as the U.N. sanctions." (Yonhap News)