The U.S. soldiers removed their gas masks and "contaminated" clothing on the icy river bank, shivering as they rehearsed for what they fear most: a chemical attack.

Other soldiers scrubbed Army trucks with high-powered hoses, while men in baggy chemical suits dumped clothes into piles to be burned.

"That's possibly the scariest thing that people fear, chemical weapons," Army Cpt. William Vickery said as his decontamination troops staged an attack simulation within artillery range of North Korea, a communist country believed to have up to 5,000 tons of chemical agents.

As the hunt intensifies for chemical weapons in Iraq, U.S. soldiers in South Korea need only look north across the heavily mined demilitarized zone to see perhaps a more formidable, yet comparatively neglected, chemical danger.

"I hope they realize that threat is real," Vickery said. "We know it's there."

Underlining the peril, the 37,000 U.S. troops in South Korea began receiving next-generation chemical suits last month. Called JSLISTs, for Joint Service Lightweight Integrated Suit Technology, they are lighter than the old suits, and their hoods can be tucked over a person's head faster, in about nine seconds.

They also have a life span of about 24 hours in a chemical environment, much longer than the six-hour duration of the old.

In theory, each soldier is supposed to be issued three suits. Yet only 50,000 new suits have arrived so far.

Information on North Korea's secretive chemical weapons programs is scarce, but the South Korean Defense Ministry estimates Pyongyang increased its stockpiles fivefold over the 1990s to 5,000 tons. The arsenal is believed to include some of the deadliest chemical weapons known: Mustard gas, tabun and sarin -- all of which can be fired atop the North's ballistic missiles or rained down on the South's capital, Seoul, in artillery shells.

The North is also believed to have stockpiled anthrax and nine other types of biological weapons, the Defense Ministry believes.

While North Korea has in the past denied having chemical weapons, a high-ranking party defector, Hwang Jang Yop, alarmed the world in 1996 by admitting the North had nuclear and chemical weapons capable of "scorching" South Korea and Japan.

As a precaution, the 15,000 family members of U.S. military personnel in South Korea are also equipped with and trained in chemical suits. And Seoul has conducted mock chemical weapons attacks, during which the streets were cleared.

Despite the dangers, experts say there is one important difference between the North's threat and that of a chemically armed Iraq -- North Korea has yet to use its weapons. Iraq unleashed its arsenal against Iran in the 1980s and then against its own Kurdish minority.

"If you look at how the United States is handling the situation of chemical weapons in North Korea and Iraq, you might think there is a double standard," said Kim Tae-hyo of the Institute for Foreign Affairs and National Security, a think tank in Seoul. "But the Iraqi experience is different because they have a history of using them."

The top concern, Kim said, is North Korea exporting its chemical know-how, possibly to terrorists or rouge states like Iraq.

In the meantime, however, Washington seems more concerned to nip the North's nascent nuclear program, before Pyongyang has a chance to build an atomic arsenal as well. The United States thinks North Korea already has one or two nuclear bombs.

Secretary of State Colin Powell visited Japan, China and South Korea over the weekend, hitting hard on the need for North Korea to abandon its atomic ambitions, but largely ignoring its lesser weapons of mass destruction.

On the front lines of South Korea, however, doing that is not an option.

"No one really takes it seriously until you're actually out there," Sgt. Harold Cosper, of Burnet, Texas, said through his bubble-eyed gas mask before lining up to be stripped down.