Recent military moves by North Korea and the United States could increase the risk of an armed confrontation -- deliberate or accidental -- in the standoff over the North's nuclear program, according to Asian and U.S. military experts.

North Korea has begun supplementing its harsh rhetoric with unusual acts by its armed forces. On Monday it test-fired an anti-ship missile off its eastern coast, rattling the inauguration of the new South Korean president, Roh Moo Hyun. Last week, a North Korean MiG fighter jet intruded into South Korean airspace for the first time in 20 years.

Earlier, North Korea threatened to abandon the armistice that ended the fighting in the 1950-53 Korean War, and took successive steps to restart facilities capable of producing material for nuclear weapons.

The United States, meanwhile, has put 24 B-52 bombers on alert for possible deployment to the Pacific island of Guam, and has moved some specialty aircraft and ships closer to North Korea, according to military analysts and reports in Japan.

Those include special reconnaissance planes used to watch for nuclear testing, E6B aircraft that can direct missile launches from submarines, and the USS Invincible, a missile-tracking ship mounted with radar capable of monitoring wide areas, the analysts said.

The United States is refusing to cancel or cut back two large-scale training exercises beginning March 4 by U.S. and South Korean forces. The month-long annual war games will involve up to 5,000 soldiers, with reservists flown from the United States to reinforce units here. They will conduct mock battles near the Demilitarized Zone that divides the two Koreas. Most of North Korea's army is poised nearby.

"The ceaseless saber-rattling staged by the U.S. in South Korea is creating an extremely tense situation where it may make a pre-emptive strike at any time," the official North Korean news agency warned Wednesday. Earlier, it threatened, "If the U.S. moves to bolster its aggressive troops are unchecked, the whole land of Korea will be reduced to ashes and the Koreans will not escape horrible nuclear disasters."

"Whenever there is a military exercise, North Korea has adamantly opposed it," said Kim Chang Su, an expert at the Korean Institute for Defense Analyses, a government-run organization in Seoul. "But this time, they may really be thinking whether Mr. Bush will attack them."

"Always, the danger is, is this [the exercises] being perceived as preparation for attack?" said Michael A. McDevitt, a retired rear admiral and the director of the Center for Strategic Studies in Washington. Or, he said, do the North Koreans believe that the real intent of the military activity is "to enhance deterrence?"

U.S. officials dismiss the North's denunciations as propaganda. They say they will not scale back the exercises because of the threats, and say the annual training is a sign the military is not changing its routine because of the new tensions.

At the same time, U.S. officials are trying to signal that the North is not being targeted militarily. "There are no armies on the march. We have no plans to invade," Secretary of State Colin L. Powell said Tuesday in Seoul.

But as tensions remain high, U.S. officials say they're keeping their guard up. "We have to watch North Korea very closely," said Navy Cmdr. Matt Brown, aboard the USS Blue Ridge command ship docked in Yokosuka, Japan. Many U.S. operations in the western Pacific "are driven by what would happen in North Korea" under different scenarios, he said.

The Navy has moved the nuclear aircraft carrier USS Carl Vinson from San Diego to the western Pacific, where it is escorted by four destroyers and a cruiser, most equipped with guided missiles. The move ensures there is no drop in armed might because of the transfer of the USS Kitty Hawk, an older aircraft carrier, to the Persian Gulf.

And last week, the U.S. Army in Korea put a 90-day extension on the one-year tours of about 2,800 soldiers. The order will guarantee that U.S. forces in South Korea remain at 37,000 while replacements are diverted to the Iraqi effort.

"Our readiness is at 100 percent, and that's what we have to keep it. Our mission is absolutely non-negotiable," said Maj. Holly Pierce, a spokeswoman for the U.S. Eighth Army headquarters in Seoul.

U.S. officials say these movements are not a build-up, which would alarm North Korea. Instead, they describe "precautionary" moves meant to deter the North while the United States focuses on Iraq. "They have done some very, very careful things not wanting to trigger a crisis that they insist does not exist," observed Kurt Campbell, of the Center for Strategic and International Studies in Washington.

But, looking at all the U.S. moves, some analysts see a modest rise in the American military punch. "There is an increase in U.S. capability in the region," said Hideaki Kaneda, a retired Japanese naval commander and now a security analyst in Tokyo.

Many North Korea experts say the United States' various moves are feeding fear among the isolated North Korean leaders that they are next on the list for attack. Iraq is about to get hit, government leaders reason, and then U.S. forces will focus on another country that the White House has declared to be part of the "axis of evil," North Korea.

But analysts generally discount the possibility that North Korea will respond by attacking first, on a grand scale, because that would be suicidal for the government there. Still, they caution, North Korea is volatile, suspicious and unpredictable. And the heightened tensions add to a danger that an overzealous military commander could act on his own.

"North Korea is walking a delicate balance, between the need to scare their own people" to divert attention to an external threat, "without doing something stupid that triggers a response," said Rep. Mark S. Kirk (R-Ill.), who closely follows Korea issues. But "there's always some idiot that doesn't get the word and creates a crisis."

But other analysts contend that the North Korean leader, Kim Jong Il, has been forced by the failure of diplomatic efforts toward Washington to give his hard-line military generals greater authority. The missile launch and MiG incursion show that the generals are anxious to test the United States by military provocations, these analysts say.

"The North Koreans have used almost all their cards," said Toshiyuki Shikata, a professor of crisis management at Teikyo University in Tokyo. "So they will try to use the final card: They will launch a missile over Japan."

U.S. soldiers from the 2nd Infantry Division take part in a military exercise in South Korea. North Korea claims such training is preparation for an attack.