The reported nuclear test was "a serious problem" and a matter of concern, the top government spokesman said in a statement.

"It is a severe threat not only to Japan's security, but also to the peace and stability to East Asia and the international society," Shiozaki said.

The chief cabinet secretary said the DPRK's action violated the Treaty on the Non-Proliferation of Nuclear Weapons (NPT), the declaration between Japan and the DPRK, as well as the joint declaration of the six-party talks on the nuclear issue of the Korean Peninsular.

The top spokesman urged the DPRK to abide by the U.N. Security Council resolution, give up all its nuclear weapons and nuclear plans and return to the six-party talks.

The foreign ministry said Japanese Prime Minister Shinzo Abe agreed with U.S. President George W. Bush in a telephone conversation that they would take decisive action against the DPRKat the U.N. Security Council over the issue.

Abe and Bush shared the view during the 15-minute talks that the DPRK's moves were "not acceptable and cause serious threats to the peace and security of the international community and pose a serious challenge to the nonproliferation regime".

The two leaders said their countries would be in close cooperation with other stakeholders including participants in the six-nation talks, such as South Korea, China and Russia, and added that the U.N. Security Council should take action against the DPRK.

Abe talked with Bush shortly before he left Seoul, capital of South Korea, for Japan on Monday evening after a summit with South Korean President Roh Moo hyun, the ministry said.