President Bush says North Korea will face "serious repercussions" for its apparent nuclear test.
Mr. Bush told reporters on Wednesday that the United States is still working to confirm North Korea's claim that it detonated a nuclear device Monday, but that the claim itself constitutes a threat to international peace and security.
The president also said the United States remains committed to diplomacy to resolve the nuclear issue. But he said Washington will work with its allies in the region to increase defense cooperation and create a ballistic missile shield.
President Bush also ruled out direct talks between Washington and Pyongyang to defuse the situation. He said bilateral talks in the past did not work.
Earlier, outgoing United Nations Secretary-General Kofi Annan called for such talks to help resolve what he called an "extremely difficult" situation.
Meanwhile, Japanese officials approved new economic sanctions against North Korea that ban all North Korean imports and ships and most North Korean nationals. Japanese officials said their country is most at risk from North Korea's long-range missiles and nuclear weapons programs.
The U.S. State Department later issued a statement in support of the new economic restrictions. The department said the move sends a clear signal to North Korea that "reckless behavior" will only further increase Pyongyang's political and economic isolation.
In South Korea, the defense minister, Yoon Kwang-ung said the military will likely "improve and enlarge" its forces to respond to a potential nuclear threat from North Korea.
North Korea said on Monday that it had conducted a nuclear weapons test. In response to U.N. Security Council deliberations about possible sanctions, Pyongyang said it considers continued pressure from the United States to be a declaration of war.
The country's official news agency, KCNA said Pyongyang will take "physical countermeasures" if Washington keeps pressing the government of Kim Jong Il over its nuclear program.
The Security Council is to meet today to discuss possible sanctions, including a total embargo on military supplies, a ban on luxury items and more financial sanctions.