U.S. intelligence agencies have detected a huge secret underground complex in North Korea that they believe is the centerpiece of an effort to revive the country's frozen nuclear weapons program, according to officials who have been briefed on the intelligence information.

The finding has alarmed officials at the White House and the Pentagon, who fear that the complex may represent an effort to break out of a 4-year-old agreement in which North Korea pledged to give up its nuclear weapons program in exchange for billions of dollars in Western aid.The finding also 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 North has said in recent months that the United States is reneging on its side of the agreement because Congress has failed to authorize tens of millions of dollars in fuel shipments for the North. The shipments are the main American contribution to a $6 billion program, under which South Korea, Japan and other nations are supposed to finance a major electric energy program as a quid pro quo for the North's abandonment of its ambitions to develop nuclear arms.

A senior administration official said the North had not yet technically violated that accord, called the Agreed Framework, because there is no evidence that Pyongyang has begun pouring concrete for a new reactor or a reprocessing plant that would convert nuclear waste into bomb-grade plutonium. The accord explicitly bars that activity.

But spy satellites have extensively photographed a huge work site 25 miles northeast of Yongbyon, the nuclear center where, until the 1994 accord, the North is believed to have created enough plutonium to build six or more bombs. Thousands of North Korean workers are swarming around the new site, burrowing into the mountainside, American officials said.

Other intelligence, which the officials would not describe, led the administration in recent weeks to warn important members of Congress and the South Korean government in classified briefings that they believed that the North intended to build a new reactor and reprocessing center under the mountain. Intelligence estimates of how long it would take to complete the project have ranged from two to six years, depending in part on how much outside help is received.

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

When Kim went to Washington in the spring he urged President Clinton to drop sanctions against the North that date from the end of the Korean War. But the new finding makes it less likely the United States will ease up on the North.

"It's a very, very serious development," said one American official, who like the others insisted on anonymity, "to say nothing of incredibly stupid, because it endangers both the nuclear accord and humanitarian aid."

Other officials said they were clearly concerned that Congress was now less likely than ever to finance the fuel oil. If Congress refused to fulfill the American commitment, the officials fear, that would, in turn, give the North an excuse to abandon the nuclear agreement officially and, perhaps, to expel inspectors from the International Atomic Energy Agency who are at Yongbyon. The North has prohibited inspectors from examining other sites outside Yongbyon.

At a meeting last week between North Korean officials and the American special envoy for Korean nuclear issues, Charles Kartman, the United States is expected to demand that the North stop all work at the new site. It is not clear whether the North already knows that its activity has been detected.

As always in matters involving North Korea, it is virtually impossible to divine the country's real intentions.

It is possible, officials say, that the mysterious North Korean leader, Kim Jong Il, is trying to rebuild a nuclear program that the West stopped in 1994. Kim may also be hoping to bolster his standing with the North Korean military. This may be a particularly critical time to placate the right wing, because Kim is expected, next month, to be given all of the titles held by his father, Kim Il Sung, North Korea's founder.