SEOUL South Korea's navy fired warning shots to drive away a North Korean fishing boat at a maritime border on Wednesday, the second incident in six days, jangling nerves in Seoul ahead of next week's G20 summit.
The South Korean capital, about 100 km (60 miles) south of the demilitarized zone dividing the peninsula, is on heightened alert ahead of the summit over concerns Pyongyang may try to create an incident to embarrass its rival.
Washington has pressed Beijing to use its influence over the North not to create an incident in the run up to the meeting.
South Korean President Lee Myung-bak told a news conference in Seoul that he was not expecting any trouble from across the border.
"I don't think the North will try to do anything when leaders of the international community are meeting to discuss the world economy," he said. "I trust the North won't do anything but still we are fully prepared."
The North this week dismissed reports it would be a "provocateur," but in the past it has staged incidents around big international events in the South.
A military officer said South Korea fired 10 warning shots to force the North Korean vessel to retreat early on Wednesday after crossing into its waters off the west coast near where one of the South's navy vessels was sunk earlier this year.
The officer said the shots were fired after repeated loudspeaker broadcasts were ignored.
It was not clear why the vessel crossed the Northern Limit Line (NLL), the disputed maritime border set unilaterally by the U.S.-led United Nations command after the 1950-53 Korean War, and why it remained in southern waters for nearly two hours.
It was the first time in seven years that the South has fired warning shots to force a fishing boat to retreat.
SEOUL ON HIGH ALERT
South Korean authorities have ramped up security for next week's G20 summit with 10,000 participants, including 32 heads of government and leaders of international organizations, expected.
Security forces have been put on high alert, anti-aircraft missiles are at the ready, shipping and air routes are under heightened surveillance and airport screening increased.
But Baek Seung-joo of the Korea Institute of Defense Analyses said North Korea was unlikely to want to escalate tensions.
"It is one thing to try to inflict damage on the South. Ahead of an international event like this, the North is not going to want to take the chance of further isolating itself from the international community," he said.
The North has in the past used high-profile international events in the South to bolster its standing, and analysts have said the North's young leader-in-waiting, Kim Jong-un, may seek to burnish his credentials with a hostile act.
Ties between the rival Koreas sank to their lowest level in decades after the South Korean corvette, the Cheonan, was torpedoed off the west coast in March, killing 46 sailors. Seoul blames the North for the attack, which Pyongyang denies.
The waters off the west coast have been the scene of deadly skirmishes in the past that have killed sailors on both sides.
On Friday an exchange of fire occurred across the Demilitarized Zone, the first such incident in years, but officials have played down the incident, saying it was likely an accident. The United Nations is investigating the skirmish.
The two incidents have occurred despite signs of a thaw in frosty relations on the divided peninsula.
The second round of reunions of families separated since the Korean War went ahead on Wednesday, bringing together people who have not seen each other in six decades for a three-day event. It was the first such event in more than a year.
The North has said it wants to resume nuclear talks, but the South has said it will not return to the negotiating table until Pyongyang shows sincerity by acknowledging its role in sinking of the South Korean vessel this year and on denuclearization.
Six-party talks, which offer the North aid for disabling its nuclear arms program, have been stalled for two years.
(Additional reporting by Yoo Choonsik and David Chance; Editing by Alex Richardson)