In this story:

• Thousands of workers on project
• Speculation on motive
• Related stories and sites

NEW YORK (CNN) -- A massive underground complex detected in North Korea could be used to revive the country's frozen nuclear weapons program, The New York Times reported Monday.

White House and U.S. Defense Department officials fear the complex is part of an effort to renege on a 1994 agreement for North Korea to abandon its nuclear weapons ambitions in exchange for billions of dollars in Western aid, officials privy to the information told the Times on condition of anonymity.

North Korean officials have said that the United States has not lived up to its end of the pact, called the Agreed Framework, because the U.S. Congress hasn't authorized fuel shipments to North Korea.

The shipments, worth tens of millions of dollars, are the crux of the U.S. contribution to a $6 billion pot to finance a North Korean electric energy program.

Spy satellites photographed a site with thousands of workers burrowing into a mountainside 25 miles (40 kilometers) northeast of Yongbyon, the nuclear center where North Korea was believed to have made enough plutonium for at least six bombs before the 1994 accord, the Times' sources said.

There is no evidence that North Korean workers have started pouring cement for a reactor or a plant that would convert nuclear waste into bomb-quality plutonium, which is barred by the accord, said a top U.S. official.

Intelligence estimates of how long it would take to complete the project ranged from two to six years, depending in part on how much outside help is received, The New York Times said.

South Korean officials played down the finding, U.S. officials said, because they feared undermining President Kim Dae-jung's "sunshine policy" toward North Korea, an effort to reopen aid and dialogue with the Stalinist government in Pyongyang.

At a meeting this week between North Korean officials and the U.S. special envoy for Korean nuclear issues, Charles Kartman, the United States will be expected to demand that the North stop all work at the new site. It was not clear whether the North already knew that its activity was detected.

It was possible, U.S. officials said, that the mysterious North Korean leader, Kim Jong Il, was trying to rebuild the nuclear program the West stopped in 1994. Kim may also have hoped to bolster his standing with the North Korean military.

This may be a particularly critical time to placate the right wing, because Kim Jong Il is expected, next month, to be given all of the titles held by his father, Kim Il Sung, North Korea's founder.

The elder Kim died in July 1994, just weeks after having defused the confrontation with the United States by telling former President Carter that he was willing to trade away the nuclear program in return for Western aid.

Another possibility was that Kim Jong Il intended to trade away the new nuclear complex the way his father traded away the last one -- if he can extract a high enough price.

American officials told the Times that building a nuclear reactor underground was an enormously difficult technical task for any country, much less one starving as a result of economic collapse, drought and floods.

Discovery of the complex by U.S. intelligence agencies follows a string of provocations by the North, including missile sales to Pakistan and the incursion of a small North Korean submarine carrying nine commandos off the South Korean coast this year.

The Associated Press and Reuters contributed to this report.

"Disarming Strangers: Nuclear Diplomacy with North Korea" by Leon V. Sigal "Technology & the Proliferation of Nuclear Weapons" by Richard Kokoski "Deterrence and Defense in a Post-Nuclear World: Emerging Soviet and American Strategies" by Gary L. Guertner "Global Engagement: Cooperation & Security in the 21st Century" by Janne E. Nolan (Editor) "The Two Koreas" by Don Oberdorfer Click here for additional reading on Nuclear Disarmament