"If that is a fact, I cannot help but say that is extremely regrettable because that violates U.N. Security Council resolutions," Hirano told a news conference.

While stressing that Tokyo is still analyzing the information, the top government spokesman said, "We cannot tolerate North Korea possessing nuclear (capabilities)."

Hirano urged North Korea to return to the six-party talks aimed at denuclearizing the country, saying that Tokyo wants Pyongyang to do so as soon as possible.

The nuclear disarmament talks, involving the two Koreas, the United States, China, Japan and Russia, have been stalled since last December.

In April, North Korea withdrew from the talks and said it would start reprocessing nuclear spent fuel rods to produce plutonium in protest against U.N. criticism of its rocket launch earlier that month.

The announcement by Pyongyang's official Korean Central News Agency, which came a day after Pyongyang repeated its call for direct talks with Washington, is seen as a move apparently aimed at drawing the United States into such talks to address the nuclear standoff.

In September, Washington announced a plan to seek direct talks with Pyongyang as part of efforts to resume the six-way talks. But the United States is still deciding when and where to have the bilateral talks, a U.S. State Department spokesman said.