The U.S. and South Korea believe it is the start of construction for an underground reactor to produce plutonium for nuclear weapons, reports CBS News National Security Correspondent David Martin.
"There is compelling evidence that this site is intended to be used for nuclear-related activities," says Charles Kartman of the U.S. State Department.
Kartman was just in North Korea trying unsuccessfully to find out whether the world's last Stalinist regime is reneging on a 1994 deal to freeze its nuclear weapons program in exchange for economic assistance.
North Korea rebuffed U.S. appeals to inspect the underground site, demanding $300 million simply for the right to look.
The deal was intended to buy time until North Korea either collapsed under the weight of famine or decided to cooperate. But Robert Manning of the Council of Foreign Relations says it's not working out that way.
"Now it looks like it may have been the North Koreans who were buying time," Manning says. "While we were feeding them, they were building new missiles and perhaps weaponizing plutonium."
North Korea already has tested a missile capable of reaching Alaska and another test is expected next month. The country fired a missile over Japanese territory in August, rattling nerves in Tokyo, Seoul and Washington. Now U.S. intelligence has detected the construction of launch sites for missiles that could be armed with nuclear warheads.
U.S. Secretary of State Madeleine Albright said Friday that Washington and the North Korean capital of Pyonyang are fast approaching a moment of truth.
"We are at a critical juncture in our relations with Pyonyang," Albright said.
If diplomacy fails to put a lid on North Korea's nuclear program, a senior military officer told CBS News that the U.S. would be prepared to start a war to prevent the North from getting nuclear weapons.
A war in Korea would, in the words of another senior defense official, would be "deadly, extensive and final."
In a visit to South Korea on Friday, President Clinton was emphasizing security after questioning whether communist North Korea was "moving toward a more hostile posture."
"We are quite concerned about some of the news," the president said. "There are some disturbing signs there."
After a late-night arrival, Mr. Clinton was to meet Saturday with President Kim Dae-jung, the longtime dissident and democracy advocate elected president last December on his fourth try.
Speaking to reporters earlier, Kim said he would urge President Clinton not to press Norh Korea too hard about the suspected nuclear site. Kim has advocated a "sunshine policy" of engagement toward North Korea.
"We need patience when dealing with North Korea," Kim said. "Without any conclusive evidence, it is not a good idea to blow up the problem."
North Korea considers the United States an enemy and remains in a technical state of war with South Korea 45 years after the Korean War cease-fire.
In a reminder of U.S. military muscle, President Clinton on Saturday is visiting a training center near the demilitarized zone and inspect combat vehicles such as Apache attack helicopters, M-1 Abrams tanks and Bradley Fighting Vehicles. He also will stop at Osan Air Base, host to U.S. Air Force U-2 spy planes that keep an eye on North Korea.
The emphasis on security here follows talks in Tokyo about Asia's economic collapse and Japan's responsibility to get its financial house in order and help lift the rest of the region.
Reported By David Martin