WASHINGTON - North Korea started to reopen a sealed plutonium reprocessing plant yesterday, the most provocative and technically important step it has taken in recent days to revive a nuclear program that experts said could produce weapons within months.

The International Atomic Energy Agency said North Korean officials had disabled surveillance cameras and broken through seals barring entry to a building housing the equipment needed to turn spent fuel rods from a nearby reactor into weapons-grade material.

On Sunday, North Korean officials had disabled cameras and broken seals around a pool holding 8,000 of the spent fuel rods at the Yongbyon nuclear complex, 55 miles north of Pyongyang, the capital. On Saturday, the North Koreans had begun dismantling monitoring equipment at the reactor.

"The reprocessing plant is the important one because that's where they extract the plutonium from the spent fuel," said Mark Gwozdecky, spokesman for the energy agency in Vienna, Austria. "If we don't have our monitoring equipment in place, we're not in a position to assure anybody that this material is not being diverted for weapons."

The Bush administration emphasized that it would continue to deal with the issue diplomatically. But even as he endorsed diplomacy as the right course, Defense Secretary Donald H. Rumsfeld warned North Korea not to assume that the United States was incapable of confronting it militarily, even as Washington prepares for possible war with Iraq.

These developments also generated new bipartisan pressure from Congress for the White House to rethink its policy of not negotiating until North Korea drops its nuclear program.

North Korea disclosed in October that it had continued to pursue a nuclear program in violation of a 1994 agreement. That announcement led the United States to cut off shipments of oil to North Korea.

Since then, Pyongyang has steadily increased the stakes in the confrontation, apparently in an effort to win economic concessions and security agreements from Washington at a time when the United States is focused on Iraq.

Faced with the new provocation yesterday, the administration said it would stick to its demand that North Korea drop its nuclear program as a condition for negotiations.

"We think it's important to let the North Koreans know that the way to engage and integrate with the international community is to live up to treaties and agreements and obligations, not to break those agreements and then ask for more in return," said a White House official.

Asked whether the United States had set off the crisis by labeling North Korea part of the "axis of evil" (along with Iraq and Iran), thus backing Pyongyang into a corner, Rumsfeld replied that the responsibility rested with North Korea's totalitarian leadership.

"The idea that it's the rhetoric from the United States that's causing them to starve their people or to do these idiotic things," Rumsfeld said, "misses the point."

Secretary of State Colin L. Powell continued the American efforts to maintain a united international front.

Yesterday morning, Powell spoke with his counterparts in Russia, France and Britain to emphasize the need for "a peaceful resolution," said the State Department spokesman, Philip Reeker. Over the weekend Powell spoke with the foreign ministers of South Korea, Japan, China and Russia.

Reeker repeated the U.S. position that there can be no negotiations while North Korea is pursuing its nuclear program. "We will not give in to blackmail," he said. But as North Korea raises the stakes, the administration faces pressure at home and abroad to reconsider its hard line.

The newly elected South Korean government, which takes office in late February, is pledged to engagement with the north and is operating in an atmosphere of strong anti-American sentiment. Japanese diplomats have been privately concerned that isolating North Korea would backfire. Yesterday, Russia's deputy foreign minister, Georgy Mamedov, was quoted in a Moscow newspaper as suggesting that the Bush administration is to blame for the growing crisis.

"How should a small country feel when it is told that it is all but part of forces of evil of biblical proportions and should be fought against until total annihilation?" Mamedov said, according to Reuters.

Reeker dismissed Mamedov's comments as "totally absurd."

Members of Congress from both parties have also started to suggest that the administration rethink its hard line against negotiations.

Sen. Joseph R. Biden Jr. of Delaware, departing Democratic chairman of the Foreign Relations Committee, said in an interview that the growing prospect of a nuclear crisis was likely to increase the pressure for international negotiations with North Korea even if they do not directly involve the United States.

On Sunday the incoming Republican chairman of the committee, Sen. Richard G. Lugar of Indiana, said in an interview with Fox News that the United States would "have to talk, talk continuously to South Korea, to North Korea, to Japan, be heavily engaged."

Rep. Curt Weldon, a Pennsylvania Republican, said he would like to travel to North Korea to establish communications with its government. "No dialogue is a recipe for disaster," he said. "That doesn't mean we have to appease or to cave."

Two Democrats, Sens. John Kerry of Massachusetts and Charles E. Schumer of New York, wrote to President Bush last week asking for an explanation of the administration's policy.

"The administration has not enunciated a clear policy goal on North Korea," Schumer said yesterday. "They say, 'Let's not talk.' But where is that leading us? You don't have to have a Ph.D. in foreign relations to understand that North Korea poses a greater danger to the United States than Iraq. But nobody quite knows what our policy is."

Although North Korea is well down the path to completing all the steps it needs to begin processing weapons-grade plutonium at Yongbyon, officials said there is time to avert a crisis.

Gwozdecky said the atomic energy agency's inspectors on the scene had reported that the North Koreans had not finished breaking through all the seals - a combination of electronic alarms and bolts - to get into the reprocessing plant, a job he said would probably be completed today. He said it would be weeks or months before the plant would be operational.

Faced with a similar impasse with North Korea in 1994, the Clinton administration considered a plan to bomb Yongbyon but instead managed to reach a negotiated settlement. But even as it has laid out a formal national security doctrine of pre-emption, the Bush administration has stressed that it is not contemplating military action against North Korea.

U.S. officials played down any sense of urgency. Responding to questions about why evidence that Iraq has weapons of mass destruction requires the threat of immediate military action while the certainty that North Korea is on the verge of being able to produce nuclear weapons does not, administration officials said Iraq had exhausted its chances to resolve its conflict diplomatically while North Korea had not.

"The situation in North Korea is a fairly recent one. The diplomacy that's under way there is in its early stages for the United States and the interested neighboring countries. It seems to me to be a perfectly rational way of proceeding," Rumsfeld said.