Washington ? The North Korea nuclear crisis deepened Tuesday as a U.N. watchdog agency said it no longer can tell whether North Korea is diverting nuclear material at its Yongbyon nuclear complex to the production of weapons.

And the leading military official in the isolated Communist country Tuesday threatened a “fight to the end” against the United States.

“If they, ignorant of their rival, dare provoke a nuclear war, the army and people of (North Korea) led by Kim Jong Il, the invincible commander, will rise up to mete out determined and merciless punishment to the U.S. imperialist aggressors with the might of single-hearted unity more powerful than an A-bomb,” said Defense Minister Kim Il Chol, according to the official Korean Central News Agency.

The International Atomic Energy Agency’s warning came as North Korean technicians continued dismantling seals and monitoring devices on a facility that extracts weapons-grade plutonium from spent fuel rods removed from a research reactor at Yongbyon.

Unsealing the reprocessing facility is the most provocative step the secretive Stalinist regime has taken toward reactivating the nuclear weapons program it froze under a 1994 agreement with the United States.

“Unless the IAEA is able to reinstate without delay its safeguards measures at these facilities, it will not be able to provide assurances that the DPRK is not diverting nuclear material to nuclear weapons or other nuclear explosive devices,” said a statement issued by IAEA Director General Mohamed ElBaradei. “This rapidly deteriorating situation in the DPRK (Democratic People’s Republic of Korea) raises grave nonproliferation concerns.”

U.S. officials continue to believe that North Korea’s actions are “a cry for help,” as one senior official put it, from a starving, impoverished and isolated nation that’s facing a bitter winter but too proud to beg for food and fuel.

The result, for now, is an impasse. North Korea says it won’t consider re-freezing its weapons program until the United States and its allies resume deliveries of fuel oil, and the United States says it won’t even discuss helping North Korea until Pyongyang stops violating the 1994 agreement.

The United States needs to break the escalating tensions soon because within six months to a year, the North Korean military will have progressed so far that “you can’t get them to give up their nuclear weapons,” said physicist and former weapons inspector David Albright, the president of the Institute for Science and International Security.

South Korean officials said Tuesday that they believe the North Koreans have already begun inspection and repair work on Yongbyon’s reprocessing facility. They estimated that it will take a few more weeks before it could be up and running.

The Yongbyon complex, located about 55 miles north of Pyongyang, comprises the 5-megawatt research reactor and storage facility for spent fuel rods, the reprocessing plant, an unfinished electricity generating plant and a fuel rod fabrication facility. Under the 1994 agreement with the United States, the reactor, some 8,000 spent fuel rods, the fuel rod fabrication plant and the reprocessing plant were placed under IAEA seals and monitoring.

In the agreement, North Korea was to receive help in building two pressurized light-water reactors to make electricity, as well as about 3.3 million barrels of fuel oil each year to help meet its energy needs. The United States and its allies halted fuel oil shipments to North Korea in October after North Korea announced it had violated the agreement by pursuing a program to enrich uranium to make nuclear weapons.

North Korea responded on Saturday by removing IAEA-installed bolts, alarms and cameras on the 5-megawatt research reactor. On Sunday, it dismantled monitoring equipment and seals on canisters holding the estimated 8,000 spent fuel rods that once powered the reactor.

Mark Gwozdecky, a spokesman for the Vienna-based IAEA, said that in addition to the removal of monitoring equipment and seals at the reprocessing plant, IAEA inspectors reported that North Korean technicians began preliminary work of undetermined purpose Tuesday on the research reactor.

He said the reactor and the reprocessing facility could be fully operational in “not less than a month, and possibly several months,” Gwozdecky said.

Once Yongbyon’s reprocessing plant is back in operation, North Korea will be able to produce enough plutonium from the 8,000 spent fuel rods for several bombs, boosting the estimated size of North Korea’s arsenal to as many as four warheads, said a senior U.S. official, speaking on condition of anonymity.

South Korea and Japan are working intensely to convince Pyongyang to step back from a possible nuclear confrontation with the United States.

The Clinton administration, facing a similar crisis in 1993-94, made the startup of the plutonium extraction facility a “red line” for U.S. military action.

Bush administration officials have refused to be pinned down on whether they have adopted the same policy, although Secretary of Defense Donald H. Rumsfeld on Monday asserted that the United States could fight simultaneous conflicts in Iraq and the Korean peninsula.

U.S. military action against Yongbyon could have potentially devastating consequences, however. North Korea could retaliate with thousands of artillery pieces that it maintains along the border within range of South Korea’s capital, Seoul.

Tens of thousands of South Korean civilians could die, and some 37,000 U.S. troops deployed in South Korea could become embroiled in a war that could go nuclear.

Many believe that North Korea’s nuclear capabilities present a greater threat to international security than that posed by Iraqi President Saddam Hussein, who has been unable to make a nuclear weapon.

But the Bush administration is continuing a military buildup for a possible invasion of Iraq, while working with South Korea, Japan, China, Russia and other countries to pressure North Korea to give up its nuclear program.