BEIJING — North Korea sparked international alarm yesterday by pledging to test a nuclear weapon, an act of brinkmanship apparently intended to blackmail America into dropping economic sanctions.

After weeks of intelligence reports suggesting the reclusive dictatorship of Kim Jong Il was moving toward testing the arsenal that it claims to have developed, its Foreign Ministry confirmed its plans.

“The U.S. extreme threat of a nuclear war and sanctions and pressure compel the DPRK [Democratic People’s Republic of Korea] to conduct a nuclear test, an essential process for bolstering nuclear deterrent,” it said.

The regime in Pyongyang regularly claims that it is about to be attacked by America and has always said economic sanctions of the sort the Bush administration is bringing to bear amount to an act of war. In a sign that the test might not be as imminent as some have feared, it qualified the threat. “The DPRK will conduct a nuclear test in a condition where safety is firmly guaranteed,” it said, seeming to confirm suggestions that, while it may have enough plutonium or enriched uranium to build nuclear weapons, it may not yet have the full technical ability to “weaponize” them.

Nevertheless, the statement’s implication that a political decision had been taken caused consternation across Asia. Japan, where Shinzo Abe, a new, hawkish prime minister, took office last week, said the threat was “totally unforgivable” and that it would “react sternly.”

Mr. Abe wants to rewrite Japan’s constitution and to allow its defense forces to strike pre-emptively, a move largely prompted by its fears of North Korea’s military.

South Korea, which has done more than any other western-allied country to try to foster entente with North Korea, called an emergency meeting of its top security committee and increased its alert level. “This poses a grave threat to peace on the Korean peninsula,” its Unification Ministry said.

North Korea’s admission of its nuclear weapons program has been overshadowed by the arguments over Iran’s covert one, particularly while a series of six-party negotiations involving the two Koreas, America, China, Russia, and Japan were supposed to find a solution. But the six have not met for a year, following a North Korean boycott in protest at American attempts to block off its sources of foreign currency, believed to include counterfeit dollar notes and drugs.

The statement demonstrates the effect those sanctions are having. But recent visitors to North Korea report that the gradual infiltration of western goods and information in recent years has made more people aware of the gulf in living standards on the two sides of the Korean demilitarized zone — and that dissent is increasing.

Attention has focused on China’s role as North Korea’s historic ally. The Beijing leadership is said to be frustrated at both the North’s nuclear provocations and its wider refusal to follow China’s lead in reforming its bankrupt economy. Although China continues to prop up Mr. Kim’s rule by providing the bulk of the country’s energy supplies as well as food aid, it also co-operated with America in freezing suspected North Korean bank accounts.

Some analysts believe that Beijing has contingency plans to send in the People’s Liberation Army to “stabilize” the country in the event of — or to prevent — a breakdown of Mr. Kim’s regime, preferring to oversee its own sort of reform rather than allow social collapse or a reunification of the two Koreas under America’s security umbrella.

A professor of international relations at People’s University in Beijing, Shi Yinhong, said the North’s statement did not mean that a test was about to happen but did suggest hard-liners were increasingly holding sway in Pyongyang. “The situation is really dangerous,” he said.