North Korea said Wednesday it had unloaded 8,000 spent fuel rods from its nuclear reactor and planned to reprocess them to make nuclear bombs.

A senior US envoy said the same day that preparations for North Korea's first ever nuclear weapons test were in the works.

"The (South Korean) government has serious concern about the aggravated situation," South Korean Foreign Minister Ban Ki-Moon told YTN television.

"I think China, Russia, the United States and Japan all have serious concerns about it."

He said reprocessing would be viewed as further "aggravating" the standoff while a nuclear test would be regarded more seriously.

"If North Korea goes ahead with a nuclear test, it will be completely isolated from the international community as a result," he said.

Analysts are divided on whether North Korea wants to build new atomic weapons or is using nuclear brinkmanship as a tool to extract concessions from the United States. But a sense of an impending crisis is brewing.

"It is quite clear we are heading to a very serious crisis," said Jun Bong-Geum, who heads the Institute for Peace and Cooperation, a Seoul think tank.

"History of the past 15 years shows that there have only been useful negotiations when matters have reached a crisis."

Seoul said the North should return to dialogue.

"North Korea should immediately return to the dialogue table and talk, rather than delay the settlement of the nuclear issue by taking unhelpful measures," said Rhee Bong-Jo, South Korea's vice unification minister.

South Korean officials believe North Korea's latest move indicates the regime is raising the stakes in a game of nuclear brinkmanship.

Last month, Pyongyang said it had shut down its nuclear reactor, a necessary step prior to unloading spent nuclear fuel. The new claim that unloading has been completed was a logical escalation.

"It is a move that has been expected since March 31, when the (North Korean) nuclear reactor was shut down," said Rhee, the number two official in the ministry that handles relations with the isolated North.

Washington warned it could take the case to the UN Security Council and called on China to take "robust" steps to bring North Korea back to the six-party talks that include their governments as well as Japan, South Korea and Russia, and which have been in limbo for almost a year.

China opposes applying force to North Korea and has reportedly rebuffed a US request to cut oil supplies to its unpredictable ally.

Japan expressed alarm at North Korea's latest claim, with Chief Cabinet Secretary Hiroyuki Hosoda telling reporters "we have extremely strong concerns".

Ban, accompanying President Roh Moo-Hyun on a visit to Uzbekistan, told reporters the latest claim looked like a bid to pressure the outside world, "because they made such an announcement in public".

North Korea has been raising the stakes at predictable intervals since February 10, when it declared itself a nuclear power and said it would stay away from talks.

In March it said it would end a missile testing moratorium and threatened to build more nuclear bombs.

Then it confirmed US reports that it had shut down its nuclear reactor in a step that could lead to the doubling of its nuclear arsenal if it reprocessed the facility's 8,000 spent fuel rods.

Fears that efforts to negotiate would evaporate completely were compounded by US ambassador to Japan Thomas Schieffer, who said he believed North Korea had taken "some preparatory steps" toward carrying out a test.

Recent media reports from the United States have quoted US officials as saying Pyongyang had been preparing an underground nuclear test since March at Kilju, in northeastern North Korea, and might conduct one as early as June.

Yim Yong-Soon, an advisor to the National Security Council, said if North Korea were making nuclear weapons, it would do so secretly.

"But they are using this as a bargaining chip," said Yim, a Sungkyunkwan University professor. "If they did ever have a bomb, they would not be stupid enough to test it."

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