Rattling nerves along the border, a North Korean fighter jet violated South Korean airspace over the Yellow Sea on Thursday before turning back as warplanes in the South scrambled. The flight -- the first such incursion in 20 years -- was the latest in a series of North Korean provocations.

The incursion, which lasted two minutes, came only days after North Korea threatened to abandon the armistice keeping peace along the border if the United States imposes sanctions on the communist regime.

The flight also underlined heightened tensions just days ahead of a visit to South Korea by Secretary of State Colin Powell to discuss the standoff over the North's nuclear program.

South Korea protested the intrusion, the first by air since 1983.

"Our military sternly protests the North Korean provocation and demands that the North take actions to prevent a recurrence of similar incidents," Defense Ministry spokesman Brig. Gen. Hwang Young-soo said.

He said the incursion "could result in very serious consequences in the current situation on the Korean Peninsula."

North Korea recently has taken a series of steps apparently designed to draw international attention in hopes of getting direct negotiations with Washington over its nuclear program.

Pyongyang made no comment on the incursion. But late in the day, North Korea's official news agency, KCNA, described the situation on the Korean Peninsula and in northeast Asia as "so alarming that a nuclear war may break out any moment."

Despite the tensions, President-elect Roh Moo-hyun said Thursday he would push for greater reconciliation with North Korea and called for a peaceful resolution to the nuclear standoff.

"North Korea's nuclear development attempt will never be tolerated," Roh told a dinner party at a Seoul hotel. "However, the problem should be resolved through dialogue and diplomatic means in a peaceful manner."

The nuclear dispute gathered pace in October, when U.S. officials said North Korea admitted having a covert nuclear program. Washington and its allies suspended fuel shipments, and the North retaliated by expelling U.N. monitors, restarting frozen nuclear facilities and withdrawing from the Nuclear Nonproliferation Treaty.

On Tuesday, North Korea threatened to abandon the armistice that ended the 1950-53 Korean War if the United States went ahead with sanctions or other actions against the communist country.

The North Korean MiG-19 jet fighter crossed a western maritime boundary over the Yellow Sea at 10:03 a.m. Thursday. The warplane flew nearly 8 miles into South Korea's airspace -- all over water -- before heading back into communist territory two minutes later.

A South Korea anti-aircraft missile unit based near the port of Incheon was given the order to be ready to fire. At the same time, two South Korean F-5E jets flew to intercept the intruder, the Defense Ministry said. Later, four more South Korean F-5E jets were deployed.

The first South Korean jets were 19 miles, or a two-minute flight, from the enemy fighter when it began retreating, said air force Col. Oh Sung-dae.

The North does not recognize the Northern Limit Line maritime border drawn up by the U.S.-led U.N. Command at the end of the Korean War.

The disputed maritime border has long been the scene of tensions.

In June, warships of the two Koreas clashed near the western sea border. One South Korean warship sank, killing six sailors and wounding 18. North Korea admitted it also suffered casualties but did not say how many.

In 1999, a series of North Korean incursions across the western sea border touched off the first naval clash between the sides since the war. One North Korean boat sank, and about 30 communist sailors were believed to have died. Several South Koreans were injured.

China hinted Thursday that the issue of the North's nuclear plans might be resolved with regional talks -- a statement that appeared to depart slightly from its insistence that the issue was a matter for only Pyongyang and Washington.

The United States has said it wants to settle the nuclear dispute diplomatically and has assured the North that it has no plans for an attack. The North has repeatedly accused Washington of intending to invade.

Powell is to travel to Japan and China in coming days before arriving in South Korea to attend Roh's inauguration Tuesday.