A North Korean fighter plane intruded into South Korea's airspace on Thursday and the communist state, again using strident rhetoric, said nuclear war could break out on the peninsula at any time. The air incursion, the first by the North since 1983, was a fresh reminder of the tensions awaiting Secretary of State Colin Powell  when he visits the region from the weekend.

North Korea's official KCNA news agency said in a commentary that "the situation on the Korean peninsula and northeast Asia is so alarming that a nuclear war may break out at any moment."

The United States, it said, was deliberately sabotaging North Korea's improvements in relations with Seoul and Japan by "fabricating" a secret Pyongyang nuclear arms program in a bid to dominate the region.

"The U.S. will get nothing from talking about 'military counteraction' against" North Korea "and maintaining a hardline stance toward it," said the commentary, issued just days before Powell visits the region.

The South Korean Defense Ministry said the South scrambled six F-5E fighters after a North Korean MiG-19 intruded into its air space. In two minutes the MiG returned across the border over the Yellow Sea.

"We firmly protest this intrusion and strongly demand the North prevent further incidents," the ministry said, calling on Pyongyang to take "responsible measures."

The violation of the Yellow Sea border near the site of two deadly naval clashes since 1999 followed a statement from the North Korean military on Tuesday threatening to abandon the armistice which ended the 1950-53 Korean War.

The Korean People's Army said it would walk away from the 50-year-old truce if economic sanctions were imposed on Pyongyang because of the four-month-old crisis over its suspected drive to manufacture nuclear weapons.

U.S. disagreements with Asian states over how to end the standoff are expected to dominate Powell's four-day trip to Japan, China and South Korea starting at the weekend.

Powell travels to Tokyo on Saturday, to Beijing on Sunday and to Seoul on Monday to attend Tuesday's inauguration of South Korean President-elect Roh Moo-hyun.

U.S.-SOUTH DIFFERENCES

The United States has sought without visible success to persuade regional powers, notably China, to press North Korea to abandon its suspected nuclear program and favors multilateral talks to urge Pyongyang to do so.

North Korea wants a non-aggression pact with the United States and has called for bilateral talks, something other countries in the region, particularly China, are believed to favor but Washington has resisted.

"There are clearly differences," said State Department spokesman Richard Boucher.

"That's why we continue to discuss how we proceed."

Roh, who in contrast to the United States has said he would rule out the use of force with North Korea, said he believed the crisis can be settled through dialogue.

"The mere hint of war and the anxiety it entails can inflict great losses upon us," he said in a speech to the Heritage Foundation, a conservative U.S. think-tank.

"We do not want war or North Korea's collapse. But North Korea must realize that it has no alternative but to open up and reform."

South Korea and its neighbors have also voiced reluctance to impose sanctions on the North -- a move the U.N. Security Council deferred on Wednesday to allow for further diplomacy.

Despite the tensions, more than 500 South Koreans traveled across the fortified frontier to North Korea on Thursday for family reunions with northern relatives they last saw 50 years ago. The meetings are a core North-South reconciliation project.

The North Korea crisis began in October, when U.S. officials said the secretive communist state had admitted to pursuing a nuclear weapons program.

It escalated over the past two months as Pyongyang expelled International Atomic Energy Agency inspectors, said it would pull out of the nuclear Non-Proliferation Treaty and threatened to resume missile testing.

While the United States has said it is willing to talk directly to North Korea about ending its nuclear program, it has ruled out offering it inducements to do so and has made clear it wants to deal with the issue on a multilateral basis.

The United States keeps 37,000 troops in South Korea under a 50-year-old security alliance. The two Koreas remain in a technical state of war because the 1953 armistice never gave way to a peace treaty.

PHOTO CAPTION

South Korean colonel Oh Sung-dae briefs the media in Seoul on a North Korean fighter plane that intruded into South Korea's western airspace February 20, 2003. The air incursion, the first by the North since 1983, was a fresh reminder of the tensions awaiting Secretary of State Colin Powell  when he visits the region this weekend. (The Dong-A Ilbo/Reuter