In the past month, U.S. spy satellites have detected smoke rising from the once shuttered buildings clustered around a loop of North Korea's Kuryong River. Trucks arrived and departed, and workers bustled.
The Yongbyon Nuclear Center is one of the most heavily guarded areas in one of the world's most secretive nations, and it is the focal point of rising tensions over North Korea's nuclear weapons program.
American analysts aren't sure what is going on there, and some South Korean experts think the North is staging phony activity as a bargaining chip in its effort to get Washington to sign a nonaggression treaty. But the increased movement at the site 50 miles north of the capital, Pyongyang, has increased anxiety over the North's intentions.
Neighboring nations worry the North may be resuming its program to produce nuclear weapons, fearing that could bring an arms race in the region or even war. The faceoff also has caused some strain between Washington and South Korea's government over how to deal with the crisis.
Experts say the complex is home to 8,000 spent nuclear fuel rods that could be processed within a few months into enough weapons-grade plutonium for several atomic bombs.
"The moment they remove those rods for reprocessing will be the moment they cross the danger line," said Paek Hak-soon, a North Korea expert at Seoul's independent Sejong Institute. "Whatever they do at Yongbyon will be carefully calculated and choreographed."
Earlier this month, U.S. officials said satellite images caught covered trucks apparently taking on cargo around the fuel rod storage facility, but they were divided over whether the North Koreans were really removing rods or just bluffing.
"They are just putting up a Potemkin village," said Kim Dong-kyu, an analyst at Seoul's Korea University, referring to a showy facade intended to divert attention. "They know there are watched by satellites."
Paek also doesn't think the North Koreans are reprocessing fuel rods. He said the North's most likely next step would be to restart the site's nuclear reactor, which can produce more spent fuel rods.
"They will save reprocessing the spent rods as an option they can use at a more critical time, like when the U.N. Security Council tries to impose economic sanctions," Paek said. "Like cutting salami in thin slices, North Korea raises the stakes step-by-step."
The International Atomic Energy Agency decided last week to refer the North Korean dispute to the U.N. Security Council as a way to put more pressure on the communist state to abandon nuclear weapons work and allow the return of U.N. monitors expelled from Yongbyon late last year.
No international journalists have visited the area, and photographs of the facilities are so rare that TV stations have used the same footage of the inside of the complex for years.
North Korea says Yongbyon was built to generate badly needed electricity. U.S. officials say the entire site is a nuclear weapons facility operating behind a peaceful facade.
The complex began with a tiny research reactor North Korea built in 1965 with Soviet help. It now has a 5-megawatt reactor and an unfinished 50-megawatt reactor and facilities for fuel manufacture. Some of the hundreds of buildings there are designated "military facilities" and have never been opened to outside inspectors.
During a 1994 showdown, then-President Clinton was close to ordering Yongbyon bombed. That crisis was defused when North Korea agreed to mothball the facilities in return for free oil and help building two new, but less dangerous nuclear power plants.
Under the 1994 accord, U.S. experts visited the complex and worked with the North Koreans to place the 8,000 spent fuel rods from the 5-megawatt reactor into airtight, stainless steel canisters. The cans were sealed, tagged and placed underwater.
The uranium-alloy rods -- 1 inch in diameter, 21 inches long and 13.7 pound each -- could yield enough plutonium for several bombs if they were put through a nearby radiochemical reprocessing lab, experts say.
The current crisis flared up in October when the U.S. government said North Korean officials had admitted pursuing a secret nuclear arms program, a charge later denied by the North.
Washington and its allies cut off oil shipments, and the North responded by reactivating Yongbyon, expelling the U.N. monitors and withdrawing from the Nuclear Nonproliferation Treaty.