Throughout January, U.S. intelligence has detected substantial activity at North Korea's once shuttered nuclear facility, a possible sign it is getting ready to produce nuclear weapons, officials said Friday.

American spy satellites have detected covered trucks apparently taking on cargo at the nuclear storage facility at Yongbyon, where spent nuclear fuel rods are stored, said U.S. officials, speaking on condition of anonymity. When processed, enough plutonium can be extracted from the 8,000 rods to make four or five nuclear weapons, U.S. officials have said.

It is possible -- but not certain -- that the trucks seen at the plant are loading those rods, either to be stored elsewhere or in preparation for processing, one official said. Another said more people have been working at the complex, North Koreans also have been grading roads and plowing snow from roads and that smoke has been coming out of smokestacks at the complex -- signs the regime in Pyongyang is resuming operations at the facility.

The activity is not particularly unexpected, since the Koreans withdrew Jan. 10 from a global anti-nuclear pact and said they would restart the reactor at Yongbyon to generate electricity.

But restarting it would be another ominous step in a crisis that has been escalating on the Korean peninsula since October, while the Bush administration has been preoccupied with launching a possible war on the other side of the world to disarm Iraq.

While there is agreement in the intelligence community that North Korea is gearing up at Yongbyon, there is disagreement on how far along they are, one official said, calling the activity detected by spy satellites "like circumstantial evidence."

The New York Times reported in Friday editions that intelligence analysts have concluded North Korea could begin producing bomb-grade plutonium by the end of March. But one official said that would be too soon, given the plant has not been in operation for years.

Washington, under President Clinton, drew up plans to bomb Yongbyon in 1994 over possible North Korean weapons activities. The two sides defused that crisis with an energy deal under which the North had agreed to freeze its nuclear facilities at Yongbyon in return for the oil shipments and construction of less threatening nuclear electricity generating plants by a consortium made up of the United States, Japan and South Korea.

The agreement collapsed in October, when the United States said North Korea had admitted developing nuclear weapons in violation of the 1994 agreement. In response, Washington suspended the fuel shipments.

North Korea in turn expelled U.N. inspectors and announced its withdrawal from the Treaty on the Non-Proliferation of Nuclear Weapons.

Defense Secretary Donald H. Rumsfeld is monitoring the development, another official said Friday. Immersed in planning for a possible military campaign against Iraq, the Bush administration has clearly shown it wants to calm the Korea dispute even as the North seeks to build the sense of crisis for negotiating advantage.

In fact, one official said Friday, because North Korea knows well that the United States is watching it, it is possible that the movements of trucks and heightened activity at Yongbyon are staged to force Americans to the bargaining table.

North Korea has been incensed by President Bush's description of the country as part of an "axis of evil" with Iraq and Iran and has demanded assurances it will not be attacked.

Despite the activity at Yongbyon, and recent North Korean militant anti-American rhetoric, its conventional forces have not made any major aggressive moves along the border with South Korea recently, Pentagon officials say.

North Korea is now moving people and equipment as part of its usual annual winter training, a senior defense official said. But U.S. military officials have not seen any buildup of forces or other signs that would indicate a threatening military posture by the reclusive communist dictatorship, according to officials who spoke only on condition of anonymity.

North Korea has more than 11,000 artillery pieces -- many within range of the South Korean capital, Seoul -- that could rain between 200,000 and 300,000 shells per hour on South Korea. U.S. officials believe the American and South Korean militaries could crush North Korea, but not without the fiercest of battles.

U.S. officials also believe North Korea has one or two nuclear weapons, as well as extensive chemical and biological weapons capabilities. The North has missiles capable of hitting South Korea, Japan and China.

On Thursday, North Korea called Bush's State of the Union message this year an "undisguised declaration of aggression." The North pledged it "will never allow the U.S. to wantonly encroach upon the sovereignty and dignity of the (North) and destroy its system."

In his Tuesday speech, Bush called North Korea an oppressive regime trying to blackmail the world with its nuclear program.

State Department spokesman Richard Boucher said Thursday the United States expects the United Nations' International Atomic Energy Agency to schedule a meeting soon to discuss referring the North Korea matter to the U.N. Security Council.