VIENNA, Austria — North Korea barred U.N. nuclear inspectors from its main nuclear reactor today and within a week plans to reactivate the plant that once provided the plutonium for its atomic test explosion, the chief U.N. nuclear inspector said.

The North ordered the removal of the U.N. seals and surveillance equipment from the Yongbyon reactor, a sign it is making good on threats to restart a nuclear program that allowed it to conduct a test explosion two years ago.

But the North’s moves could be motivated by strategy as well. It could use the year it would take to restart the North’s sole reprocessing plant to wrest further concessions from America and other nations seeking to strip it of its atomic program.

Coming amid reports that that leader Kim Jong Il suffered a stroke, the nuclear reversal has fueled worries about a breakdown of international attempts to coax the North out of its confrontational isolation with most of the rest of the world.

North Korea officials have “informed the IAEA inspectors that they plan to introduce nuclear material to the reprocessing plant in one week’s time,” a statement citing the chief of the International Atomic Energy Agency, Mohamed ElBaradei, said.

The statement said he told the the IAEA board that — acting on a North Korean request — his inspectors removed all agency seals and surveillance equipment from the reprocessing plant and its immediate area, in “work that was completed today.”

Mr. ElBaradei also said the North Koreans barred the IAEA inspectors from further access to the plant.

North Korea in recent days had already signaled it would break out of a six-nation disarmament-for-aid deal, announcing that it was making “thorough preparations” to start up Yongbyon.

Inside the board meeting, the chief American delegate to the IAEA, Gregory L. Schulte, called the “moves to halt and reverse disablement … unsettling.” And in Seoul, a Foreign Ministry spokesman, Moon Tae-young, expressed deep concern.

But their comments were measured, reflecting fears that harsh condemnation would backfire by accelerating the North’s move to restore its nuclear capacities.

The agency has been monitoring the nuclear facilities at Yongbyon, which were shut down and then sealed as part of a North Korean pledge to disable its nuclear program. That move was meant to be a step toward eventually dismantling Yongbyon in return for diplomatic concessions and energy aid equivalent to 1 million tons of oil under a February 2007 deal with South Korea, America, China, Russia, and Japan.

The accord hit a snag in mid-August when America refused to remove North Korea from its list of states that sponsor terrorism until the North accepts a plan to verify a declaration of its nuclear programs that it submitted earlier.

The Yongbyon plant was under IAEA seals in December 2002 when the North decided to order IAEA inspectors out of the country and to restart its nuclear activities, after the unraveling of a deal committing America to help the country build a peaceful nuclear program.

North Korea subsequently quit the Nuclear Nonproliferation Treaty in January 2003 and announced it had nuclear weapons a little more than two years later.

A U.N. official who demanded anonymity for divulging confidential information said today that other nuclear sites in North Korea remained under IAEA purview. She also said agency seals remained on the spent fuel rods that were removed from Yongbyon under the terms of the deal.

The fuel rods are key to producing the plutonium the North would need to restart its weapons program by separating the fissile substance out of the material released once the rods are dissolved within the reactor.

The U.N. official said the three-member IAEA team was expecting that the North Koreans would also soon ask the team to remove the seals from the thousands of fuel rods in storage. More than 60 percent of those rods had already been removed under the six-nation deal.

North Korea had agreed in February 2007 to begin dismantling its nuclear program in exchange for aid and other concessions.

Scientists began disabling its reactor in November, and in June blew up the Yongbyon cooling tower in a dramatic show of its commitment to the pact. Eight of the 11 steps needed to disable the reactor were completed by July, North Korean officials said.

But later that month, Washington made an additional request: detailed verification of the process, including soil samples and interviews with scientists. America pinned one of its concessions — removing North Korea from its list of nations that sponsor terrorism — on verification.

North Korea rejected the demand, saying verification was never part of the deal, and threatened to pull out of the pact, if Washington continued pressing for verification.

A North Korean envoy confirmed on Friday that authorities had stopped disabling Yongbyon and intended to restart the facility.

Experts say it would take about a year to restart the Yongbyon facilities after completely disabling it. Scientists reportedly have tested the reactor’s ignition, and this week asked the IAEA to remove its seals.