ASHINGTON, Jan. 30 ó American spy satellites over North Korea have detected what appear to be trucks moving the country's stockpile of 8,000 nuclear fuel rods out of storage, prompting fears within the Bush administration that North Korea is preparing to produce roughly a half dozen nuclear weapons, American officials said today.

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Throughout January, intelligence analysts have seen extensive activity at the Yongbyon nuclear complex, with some trucks pulling up to the building housing the storage pond. While the satellites could not see exactly what was being put into the trucks, analysts concluded that it was likely that workers were transporting the rods to another site, either to get them out of sight, or to move them to a reprocessing plant to convert them into bomb-grade plutonium.

The Bush administration has said nothing publicly about the truck activity, deflecting questions about the subject. American intelligence analysts have informally concluded that the movement of the rods, combined with other activity that now appears to be under way at the Yongbyon complex, could allow North Korea to begin producing bomb-grade plutonium by the end of March.

"There's still a debate about exactly what we are seeing and how provocative it is," said one senior official. "The North Koreans made no real effort to hide this from us."

The satellite photographs of the truck activity have been tightly held by the administration, and not yet shared widely with allies. The administration's lack of public expressions of alarm contrasts sharply with its approach to Iraq, which the head of the International Atomic Energy Agency, Mohamed ElBaradei, noted today was years behind North Korea in nuclear ability.

Some administration officials have said they want to avoid creating a crisis atmosphere with North Korea ó they believe its leader, Kim Jong Il, is hoping to set off a crisis to extract concessions from Washington ó while others say President Bush does not want to distract international attention from Iraq.

But a former nonproliferation official in the Clinton administration, Robert J. Einhorn, said tonight that by moving toward production of actual weapons, "the North Koreans may be taking a fateful step."

The spent fuel rods have been in secure storage since 1994, under a nuclear freeze agreement struck with the United States that year. But after American officials confronted Mr. Kim's government last October with evidence that it was violating the agreement by pursuing a new, clandestine nuclear program, North Korea renounced the 1994 agreement. It threw out international inspectors on New Year's Eve, and now appears to be moving the rods.

Despite the uncertainty, there is a growing consensus in the administration that North Korea is working to produce bombs as quickly as it can, perhaps hoping this will give it more negotiating leverage once Iraq is out of the spotlight.

The satellite evidence may present the Bush administration with an excruciating military choice. Pentagon officials say the North Korean program could be set back for years with a precision strike on the reprocessing plant. The plant is above ground and away from population centers. Such a strike is part of the Pentagon's contingency plan for an outbreak of hostilities on the Korean Peninsula. The Clinton administration developed plans for a strike against the complex in case diplomacy failed in the 1994 nuclear crisis.

But such a strike would be enormously risky. American officials and their allies fear that North Korea would retaliate against South Korea or Tokyo, an attack that could result in tremendous casualties.

Mr. Bush has pledged in recent weeks that "we have no intention of invading North Korea." But the word "invading" appears, to some Korea experts, to have been carefully chosen, so that Mr. Bush was not taking off the table the threat of a strike on the plant.

On the other hand, administration officials have said such a strike may not accomplish much, because North Korea has now admitted to a second nuclear program, involving enriched uranium. "We don't know where that program is," said one senior official. "So if you hit the plutonium plant, they would just speed up on the uranium program."