WASHINGTON -- Almost daily now, North Korea warns that it will regard as a declaration of war any economic sanctions imposed over its nuclear program.

No one knows whether these threats are serious or simply a bargaining strategy meant to obtain concessions from the United States and its allies in return for Pyongyang's agreement to limit the spread of nuclear weapons.

But with 37,000 U.S. troops on the front line between North and South Korea and hundreds of thousands more needed to win a war that could turn nuclear, the warnings are being taken seriously here.

U.S. officials stress there is no indication -- apart from the bellicose rhetoric -- of any imminent attack from the North. But they also acknowledge that an invasion could be launched with little preparation because North Korean assault forces are massed on alert just across the border.

Should the North Koreans carry out their war threat, it would unleash a conflict for which both sides have prepared since they clashed four decades ago.

"This potential field of battle has been more carefully studied by both sides than perhaps any other place in the world," said Loren B. Thompson, deputy director of Georgetown University's National Security Studies Program.

"It's hard to imagine a place . . .where more thought has been given by more military professionals to the way the conflict might unfold. It means we cannot be taken by surprise."

After North Korea invaded the South June 25, 1950, the war lasted three years. About 2 million troops and 2 million civilians on both sides were killed. The United States lost 37,000 troops, with an additional 103,000 wounded.

An all-out war now could leave 400,000 U.S. and South Korean troops dead, Gen. Gary E. Luck, commander of U.S. forces in South Korea, estimated earlier this year.

The Korean War ended in armistice, with a demilitarized zone between North and South Korea near the 38th parallel.

It is across that line that the 1 million-plus North Korean force now faces 650,000 South Koreans and the 37,000 U.S. troops.

The Pentagon is upgrading U.S. and South Korean weaponry, positioning supplies at sea, and has sent a battalion of Patriot anti-missile missiles to South Korea.

Although an attack by the North would be launched against South Korea, the United States would immediately be embroiled in the fight.

As General Luck, the commander in Korea, recently told Congress: "The U.S. is at war when the aggressor's first round is fired."

According to the Defense Intelligence Agency, if President Kim Il-sung, the Communist dictator who has ruled North Korea since World War II, chooses war, he is likely to launch a sudden attack to try to neutralize the South Korean and U.S. forces near the demilitarized zone, derail U.S. reinforcement efforts and isolate Seoul, the South Korean capital -- all within seven days. He would then seek peace negotiations on his terms.

U.S. suspicions

U.S. intelligence officials suspect that the North Koreans already have one or two nuclear devices, in defiance of their 1985 signing of the Nuclear Non-Proliferation Treaty, which forbids the manufacture or acquisition of "nuclear weapons or other nuclear explosive devices."

The North Koreans also have Scud missiles capable of hitting Seoul and are developing missiles able to deliver nuclear or chemical warheads 1,000 miles, within range of Japan.

It is the prospect of the North Koreans' being able to build and deliver nuclear warheads, and possibly export nuclear ballistic missiles to Third World countries, that so alarms the Clinton administration and has convinced it that the danger should be faced now, not later.

Inside the Pentagon, a new Korean war is viewed as a four-phase operation for the allies:

* Phase One -- Halt the North Korean attack, which would open with air and artillery bombardment and be spearheaded by hundreds of T-62 and T-55 tanks and thousands of infantry pouring south across the demilitarized zone.

South Koreans' role

The South Koreans, dug in 10 miles and 15 miles south of the demarcation line, would be expected mainly to block the invasion until the United States could apply air power.

The immediate goal would be to protect Seoul, just 35 miles from the front line.

To help repel the North's armor, the United States has replaced a battalion of Cobra helicopters in South Korea with more heavily armed Apache attack helicopters. A second battalion of Apaches is slated to be shipped.

The 211 1986-model M2 Bradley fighting vehicles of the 2nd Infantry Division have been replaced by more powerful M2-A2s.

The Air Force's 51st Fighter Wing, based at Osan, has 28 F-16 attack fighters and 10 OA-10 observation and forward control planes.

An additional 53 F-16s are with the 8th Fighter Wing at Kunsan air base. South Korea is also buying F-16 combat and attack jets.

Prime allied targets in Phase One, according to the Pentagon blueprint for regional conflict, would be North Korea's advancing armored and infantry columns, command and control sites, communications centers and air defenses.

According to the DIA, North Korea is suspected of having dug up to 20 tunnels beneath the demilitarized zone, each capable of allowing an infantry regiment to emerge behind allied lines within an hour. Four tunnels have been located and destroyed.

The North Koreans also have units trained in sabotage and terrorism, and their amphibious forces would be likely to open a second front with a landing south of Seoul.

* Phase Two -- Build up U.S. combat strength.

This calls for reinforcements -- planes, heavy armor, Marines and Army Special Forces -- dispatched from Japan, Okinawa and the United States within weeks.

North Korean targets

During this phase, North Korean forces would be hit by allied artillery, missile and air strikes. Prime targets would be North Korean air and naval bases and supply depots. Air strikes would reach deep inside North Korea. Psychological warfare would be unleashed.

* Phase Three -- Counter-attack.

U.S. and South Korean forces would launch land, sea and air assaults to destroy North Korea's "centers of gravity" and war-making capabilities.

L * Phase Four -- Provide post-war stability on the peninsula.

Doubts about success

Although Pentagon officials say they are confident of victory, Bob Gaskin, who was Korean adviser to former Defense Secretary Dick Cheney, believes that the South Korean defenses would be overrun too quickly for the United States to be able to apply effective reinforcements; that airfields in both South Korea and Japan would come under immediate missile, chemical and commando attack; and that allied control of the air would be hampered by the hazy atmosphere over Korea.

"The fact is, the South Korean army is extremely vulnerable to North Korean attack," said Mr. Gaskin, now with Business Executives for National Security, a Washington think-tank. "The [37,000] U.S. forces are on the margins, and we won't make a difference either way. There's too much force."

He estimated it would take at least three months for the United States to build up an effective counter-attack force.

"Even against Iraq, it took six months," Mr. Gaskin said, adding that Kim Il-sung had given his military commanders copies of the CNN news broadcasts of the Persian Gulf war to show them the U.S. weaponry and tactics they would likely face.