SEOUL, South Korea (AP) _ A North Korean fighter jet briefly crossed the western sea border with South Korea on Thursday but retreated without incident when two South Korean jets raced to the area, the South Korean Defense Ministry said.
The provocation, which also prompted the South to put an anti-aircraft missile unit on battle alert, came only days after North Korea threatened to abandon the armistice keeping peace along the countries' tense border.
The incursion, the first by a North Korean military jet since 1983, increased jitters on the Korean Peninsula, where the North is locked in a dispute over its nuclear weapons development.
``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 in a statement.
Tension has crept up since October, when U.S. officials said North Korea had admitted having a covert weapons program. Washington and its allies suspended fuel shipments, and the North retaliated by expelling U.N. monitors, taking steps to restart 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 the borderline at 10:03 a.m. (8:03 p.m. EST) Thursday and flew 7.8 miles into Southern airspace before heading back into communist territory two minutes later.
A South Korea anti-aircraft missile unit based near Incheon, a seaport west of Seoul, was ordered to be ready to fire. At the same time, two South Korean F-5E jets went to the scene to try to intercept the intruder, the ministry said. Later, four more South Korean F-5E jets were deployed to the area.
The first South Korean jets were 19 miles, or a two-minute flight, from the enemy jet when it began retreating, said air force Col. Oh Sung-dae.
The North does not recognize the so-called Northern Limit Line, a maritime border that was drawn up by the U.S.-led U.N. Command at the end of the Korean War. North Korea has often provoked armed clashes along the zone in apparent attempts to raise tensions and bring South Korea and its U.S. ally to the negotiating table.
Later in the day, North Korea's official state news agency, KCNA, described the situation on the divided Korean peninsula and in northeast Asia as ``so alarming that a nuclear war may break out any moment.''
In June last year, warships of the two Koreas clashed near the western sea border. One South Korean warship sank, killing six sailors and wounding 18 others. North Korea admitted that 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.
International debate over North Korea's nuclear plans moved ahead on Thursday, with China hinting the issue might be resolved with regional talks _ a statement that appeared to depart slightly from its weeks of insistence that the issue was a matter for only North Korea and Washington.
A day earlier, the U.N. Security Council referred the issue to experts for further study before the council begins debating the matter.
Thursday's incursion came despite continuing efforts at reconciliation between the two Koreas, which have been split since 1945. A group of South Koreans headed north on Thursday for temporary reunions with family members in the North whom they haven't seen in a half century.
South Korea has also worked hard to quell problems with the North. President-elect Roh Moo-hyun said Wednesday that he opposed any U.S. military action to force Pyongyang to abandon its nuclear programs.
``An attack on North Korea could trigger a war engulfing the entire Korean Peninsula,'' Roh said. ``It's a serious issue, and at this moment I am against even consideration of such an option.''
The United States has said repeatedly it wants to settle the dispute diplomatically and has assured the North that it has no plans for an attack.
The North, however, frequently accuses Washington of plotting a massive invasion and has warned that only direct talks with the United States on a nonaggression pact will resolve the standoff.
U.S. Secretary of State Colin Powell will travel to Japan, China and South Korea in the coming days to compare notes on the developing crisis in North Korea and to attend Roh's inauguration next Tuesday.
The two Koreas share the world's most heavily armed border. About 37,000 U.S. troops are stationed in South Korea, a legacy of the Korean War.