WASHINGTON -- The apparent shutdown of a nuclear reactor in North Korea is raising concerns among Bush administration officials that Pyongyang has completed the task of producing spent fuel rods laced with weapons-grade plutonium.
But a US official familiar with the situation said there could be at least two other possibilities, neither of which is troubling: that the reactor has run into mechanical trouble or that North Korea is bluffing in order to raise anxieties.
In the past, North Korea has claimed to have taken major steps in its pursuit of a nuclear weapons arsenal, and only some of those claims are credited by US analysts as genuine. Even so, North Korea is believed to have already produced at least one atom bomb and the United States, China, Japan, South Korea, and Russia are trying to negotiate elimination of the nuclear weapons program.
North Korea had agreed to return to the bargaining table last September after a three-month hiatus but since then has refused to resume the six-nation talks.
Reflecting growing impatience, White House spokesman Scott McClellan said the administration would consult with its partners about taking the issue to the UN Security Council if the talks remain sidetracked.
''That's one possibility," McClellan said, without offering any timetable for action.
At the State Department, spokesman Richard Boucher said, ''They need to come back to talks if they are going to solve their problems, and that's where our focus remains."
The shutdown of the reactor in North Korea's main nuclear complex at Yongbyon was detected by what US analysts refer to as ''overhead imagery," which could involve spy satellites, but not always.
The idea is to look for cessation of smoke or for significant changes in the readings of thermal or radar counts, said a US official speaking on condition of anonymity.
''This is entirely feasible, but for us to know for sure, we'd have to be physically there ourselves and this is no longer the case," said Melissa Fleming, a spokeswoman for the International Atomic Energy Agency.
In Seoul, meanwhile, Kim Sook, director general of North American affairs at South Korea's Foreign Ministry, told KBS Radio that a shutdown of a nuclear reactor at Yongbyon had been confirmed.
Yongbyon houses a reactor that generates spent fuel rods laced with plutonium, but they must be removed and reprocessed to extract the plutonium for use in a weapon. Fleming confirmed that rods can be removed only if the reactor is shut down.
North Korea restarted the reactor after expelling UN monitors at the end of 2002. Its refusal to resume negotiations has provoked behind-the-scenes quibbling among the United States and its partners over bargaining tactics.