North Korea declared Tuesday that it would conduct a nuclear test to bolster its defenses against the United States, raising tensions in the region and marking the communist government's first unambiguous pledge to prove it has become a nuclear power.

Though North Korea has previously said it possesses nuclear bombs -- U.S. intelligence officials have estimated it could have as many as 11 -- a test detonation would dramatically change the region's power dynamics. Analysts have said the United States and area neighbors including China, Japan and South Korea would be forced to deal far more harshly with the North Koreans.

A test would be a "very provocative act," Secretary of State Condoleezza Rice said during a visit to Cairo. It would create a "qualitatively different situation on the Korean Peninsula" that would spill over into the entire region, she said. Rice declined to predict what the U.S. response might be.

In a statement issued through the official KCNA news service, North Korea's Foreign Ministry said the government would "conduct a nuclear test under conditions where safety is firmly guaranteed." The statement did not say when the test might occur, but added that the North's "nuclear weapons will serve as reliable war deterrent for protecting the supreme interests of the state and the security of the Korean nation from the U.S. threat of aggression."

The declaration follows reports in recent months, based on intelligence agency findings, that the secretive communist state might be preparing a test site in its barren northeast. Observers greeted the reports with some skepticism, partly because North Korea is widely known for brinkmanship.

High-level officials from the United States, Japan, South Korea and China immediately began exchanging calls Tuesday to discuss a response, according to Asian diplomatic sources. These countries have been part of six-party talks attempting to persuade North Korea to give up its nuclear program. Large numbers of intelligence analysts and policymakers who usually split their time among a host of issues, including Iran, were devoted Tuesday exclusively to the North Korean statement.

The reaction was particularly sharp in Japan, which sees itself as a primary target of North Korean aggression. "If they conduct a nuclear test, it will not be forgiven," Japan's new prime minister, Shinzo Abe, told reporters Tuesday night. "The international community will deal with the situation firmly."

Before his election last week, Abe suggested that Tokyo should study whether Japan's constitution would allow a preemptive strike on North Korean missile bases.

Several analysts and diplomats said a test would in effect mean that North Korea's absolute ruler, Kim Jong Il, had played his last card in the standoff over the country's nuclear program. Observers suggested that the threat might be an attempt to force an easing of the economic pressure against the North that has risen dramatically in recent months.

Analysts said the threat might also reflect attempts by Kim to appease the North Korean military, which is still smarting from a failed test of its new intercontinental Taepodong-2 missile in July.

"North Korea's final goal is survival, and a test is their final option," said Ahn Yinhay, professor of international relations at Korea University in Seoul. "Given the current situation -- the enormous pressure from the U.S.'s hard-line policy -- the North Koreans may think they have no other means to try to get out of this deadlock. They may think they have nothing else to lose."

Many observers say there would be a great deal to lose. A nuclear test would make it far more difficult for Pyongyang's chief benefactors -- China and South Korea -- to continue to provide billions of dollars worth of economic aid and trade, money that has helped Kim prop up his government.

[On Wednesday, a Chinese Foreign Ministry spokesman said China "hopes that the North Korean side will keep calm and restrained on the nuclear test issue," while South Korean President Roh Moo Hyun called for a "cool-headed and stern" response, the Associated Press reported.]

Rumors of a nuclear test have surfaced almost every year since 2002, when North Korea began an escalating series of actions regarding its nuclear program -- including kicking out U.N. weapons inspectors and reprocessing spent fuel rods.

A sense of new urgency has been building in diplomatic circles in recent months. Last week, Christopher R. Hill, the Bush administration's special envoy to the stalled six-country talks, publicly warned that a test would be viewed by world powers as a "very serious" escalation.

The last country to conduct a nuclear test was Pakistan, in 1998, when it detonated five bombs underground in response to tests two weeks earlier by India.

Some scientists question whether North Korea has the technology to conduct a test safely -- leaking radiation is a major concern.

Lee Sang Deuk, vice speaker of the South Korean National Assembly, said in an interview in Washington last month that in late August intelligence agencies had detected unusual movements of troops and equipment around a complex of caves believed to contain a nuclear test site. Cables were laid and tents erected. A U.S. official confirmed that there had been signs of an impending test other than Tuesday's statement.

North Korea has frequently provided the world with early warnings of dramatic moves and has often chosen symbolic dates for them. A series of missile tests took place during the July 4 weekend, and Tuesday's statement came as South Korea was marking its national day.

"A decision to test is a political calculation, not a technical one," said one intelligence officer. The officer said analysts believed that the Pyongyang government issued the statement in part to gauge international reaction before making a final decision. "It doesn't mean they won't test, but it gives them a chance to roll back, or be coaxed back if they want to," said the officer, who agreed to discuss some of the classified analysis on condition of anonymity.

A U.S. government official said the North Koreans might also be trying to refocus international attention on their issues at a time when Iran is becoming the administration's main concern.

A major concern among U.S. officials is the potential reaction by Japan. U.S. nuclear analysts have worried for years that a North Korean nuclear test might lead Japan to break with decades of nonproliferation commitments and speed toward its own weapons capability. Japan's nuclear industry is highly advanced, giving the country the ability to make nuclear weapons within months if it chose to do so.

Frederick Jones, spokesman for President Bush's National Security Council, shared White House concerns in prepared comments that said a test "would severely undermine our confidence in North Korea's commitment to denuclearization and to the Six Party Talks and would pose a threat to peace and security in Asia and the world."

North Korea's nuclear capabilities have grown significantly during Bush's tenure. When Bush came into office six years ago, intelligence agencies estimated that North Korea had the capability to make one or two nuclear weapons. Those sources now put the number as high as 11.

State Department spokesman Sean McCormack called North Korea's announcement an "unacceptable threat" to peace and stability in Asia and in the wider world. The Bush administration will continue to work with its allies in the six-party talks to discourage "such a reckless action," the statement said.

Linzer reported from New York. Staff writer Robin Wright, traveling with Rice, contributed to this report.