North Korea on Friday announced its intention to expel U.N. inspectors from its Yongbyon nuclear reactor complex and said it would reopen a factory that extracts weapons-grade plutonium, sharply escalating its confrontation with the United States while leaving the world guessing about events in the reclusive Communist country.

In a letter sent to the U.N. nuclear watchdog, the International Atomic Energy Agency, North Korea said that because "our freeze on nuclear facilities has been lifted, the mission of IAEA inspectors has naturally drawn to an end," according to North Korea's official press. "Our government has decided to send them out."

The letter added that North Korea plans to reopen its plutonium extraction factory to provide "safe storage" for spent fuel rods taken from the reactor. Those fuel rods contain plutonium that, once it is extracted, can be used to make nuclear weapons.

In a swift reply, also by letter, the U.N. body insisted that the inspectors remain in the country to ensure that North Korea complies with its 1994 agreement with the United States that it would not develop nuclear weapons. The agency's director general asked the North Korean government to "inform him immediately should they have a contrary view, so that, if necessary, arrangements for the departure of IAEA inspectors can be made," the IAEA said in a statement.

The White House denounced the planned expulsions and urged the North to end its nuclear weapons program. "We will not respond to threats or broken commitments," said spokesman Scott McClellan in Crawford, Tex.

North Korea's latest moves were seen by arms control experts as predictable if alarming steps in its path of escalation with the Bush administration, a tactic aimed at forcing the United States to resume aid and pursue diplomatic relations. But if North Korea follows through on restarting its reprocessing plant -- the factory where plutonium is extracted from fuel rods -- that would be interpreted by its neighbors and the United States as a far more serious threat than any so far, South Korean and Western officials said. Though the United States has insisted that diplomacy is its favored means of resolving the crisis, a contrary course by North Korea would increase pressure on the Bush administration to consider a military response or at least threaten one, these sources said.

Extracting plutonium "really is crossing the red line," said Han Sung Joo, who served as South Korea's foreign minister during the outbreak of nuclear brinkmanship on the Korean Peninsula eight years ago. "If they go ahead and do that, that's really playing with fire."

Some 8,000 spent fuel rods are being stored in a cooling pond adjacent to the reactor, according to the IAEA. They contain enough plutonium to produce three to six nuclear weapons, said Shin Sung Taek, a nuclear expert at the Korea Institute for Defense Analyses, a research group in Seoul affiliated with the Defense Ministry. If North Korea restarted the reprocessing plant and began extracting plutonium from the fuel rods, it could manufacture those weapons in as little as five to nine months, Shin said.

The Yongbyon complex is 55 miles north of the North Korean capital, Pyongyang.

"This puts Washington at a crossroads," said Kim Tae Woo, an arms control expert at the Korea Institute for Defense Analyses. "They either need to heighten their level of threat to North Korea or come down to the table. This situation cannot be left to go on endlessly."

Given that South Korea and perhaps Japan fear they could be devastated by counterstrikes in any U.S. attack on North Korea, analysts have generally ruled out that option. But Friday's actions appear to have escalated the crisis. "The tension has risen to the degree where a military attack by the U.S. would not be inconceivable," said Tsutomu Nishioka, an analyst at the Modern Korea Institute in Tokyo.

The United States had assumed that North Korea would eventually resume its reprocessing operation, according to a Western diplomat who spoke on condition of anonymity. "All the signs are that they are moving beyond merely powering up and putting in good order" the reactor, "and they are in fact focusing on all the facilities at Yongbyon," the diplomat said Friday, before North Korea's announcements.

Still, he added that reviving the reprocessing plant would amount to "a very serious, serious development -- the most serious in a series of developments. Reprocessing is, in a certain sense, in a realm by itself. It is a step above and beyond." He declined to predict how the United States would respond to such an eventuality.

Arms control experts said such a course would almost certainly prompt the IAEA to file a complaint with the U.N. Security Council asserting that North Korea has violated its commitments under its agreement with the Clinton administration to abandon the development of nuclear weapons. The Security Council could then issue a warning or impose consequences ranging from economic sanctions to military force. An IAEA spokesman said the decision to take a complaint to the Security Council would be up to the agency's board of governors.

In a sign of the region's growing unease, South Korea's president-elect, Roh Moo Hyun, assailed North Korea's continued defiance, saying it jeopardized his ability to continue his country's "sunshine policy" of engagement after he takes office in February. Roh was elected on the strength of his calls to continue South Korea's moves toward reconciliation with North Korea.

"Whatever North Korea's rationale is in taking such actions, they are not beneficial to peace and stability on the Korean Peninsula and in Northeast Asia, nor are they helpful for its own safety and prosperity," Roh said in a statement.

Japan's foreign minister, Yoriko Kawaguchi, assailed North Korea's decision to expel the U.N. inspectors, saying it violated international agreements and raised "grave concerns" about nuclear nonproliferation. Japan's deputy cabinet secretary, Shinzo Abe, said North Korea "is playing an extremely dangerous game."

Friday's actions were the latest outgrowth of the unraveling of the deal that settled the previous nuclear standoff on the Korean Peninsula. Under its terms, North Korea would abandon its nuclear aspirations and submit to continuous inspections in exchange for shipments of fuel from the United States and its allies. But following disclosures in October that North Korea had secretly pursued production of uranium-enriched nuclear weapons at another site, the Bush administration halted the aid. In response, North Korea began reactivating Yongbyon.

President Bush has flatly ruled out any dialogue unless North Korea first abandons its pursuit of nuclear weapons, not only at Yongbyon but also at the uranium-enrichment site. North Korea has refused such demands, saying it would consider such a step only if the United States proffers a guarantee of nonaggression.

Last weekend, North Korea began to resume activity at the reactor complex. First, it dismantled U.N. surveillance cameras while removing seals that had validated the continued closure of the facilities. Later, it said it would revive the reactor -- not to make weapons, it emphasized, but to produce electricity. That claim was pilloried by South Korea and the United States, which argue that weapons production is the only purpose of the reactor complex.

On Thursday, North Korea moved fuel rods into the area of the 5-megawatt reactor, the heart of the Yongbyon complex, in preparation for restarting it. And Friday it took steps to ensure that the world can no longer see what it is doing there.

While most continue to interpret these actions to be levers being pulled in a negotiation, some call that view naive, noting that North Korea has genuine fears about U.S. intentions, particularly after Bush labeled it part of an "axis of evil" with Iran and Iraq.

"North Korea is intent on succeeding in making nuclear weapons," said Satoshi Morimoto, a national security expert at Takushoku University in Tokyo and a former Defense Agency official. "It's not just their diplomatic card. Once North Korea has nuclear weapons, or makes others believe that it does, the U.S. cannot attack."

North Korea's neighbors continue to try to persuade it to pull back from its nuclear brinkmanship, according to foreign diplomats. South Korea has been engaging the North through a series of such informal channels as an economic cooperation committee. Britain has communicated with North Korea through its embassy in Pyongyang, according to a British diplomat. China and Russia say they have been holding discussions as well.

Even the United States has been maintaining a "New York channel" with North Korea, according to a Western diplomat. North Korea's deputy ambassador to the United Nations holds regular conversations with the State Department's country director for Korea affairs, the diplomat said.

The United States has counted on other governments to pressure North Korea to pull back. The Bush administration views China as central to this effort. "The Chinese, in a certain sense, are the only game in town," said the Western diplomat. "They are the North Koreans' lifeline for food and fuel."

But Friday came the latest signs that the United States may not enjoy the support it needs. In Beijing, the official China Daily lashed out at Defense Secretary Donald H. Rumsfeld for comments this week in which he asserted the United States was prepared to wage successful wars in both Iraq and North Korea if necessary.

"This is a hawkish and dangerous warning," the English-language newspaper said. "It will poison the warming relations between the two sides on the Korean Peninsula."

Meanwhile, Russia accused the United States of sparking the crisis by halting fuel shipments to North Korea.

Special correspondents Sachiko Sachimaki and Akiko Kashiwagi in Tokyo contributed to this report.

The top of a reactor core at the Yongbyon nuclear plant is shown in this International Atomic Energy Agency image taken in 1992. North Korea said it would revive the reactor -- not to make weapons, it said, but to produce electricity.