South Korea says its patrol boats fired on North Korean naval vessels after they crossed a disputed maritime border.

In the latest incident along the tense frontier, the South Korean navy says its ships fired several warning shots after three boats from the North crossed the Northern Limit Line.

South Korean defense officials say the North Korean navy vessels did not respond to radio warnings to back off. They say the North's boats retreated after nearly an hour without firing a shot in return.

South Korean officials say there were about 80 Chinese fishing boats in the area at the time.

The Yonhap news agency reports that the North Korean vessels told South Korean authorities via radio that they were chasing illegal fishing boats and denied crossing the border.

The incident comes just days after holes were discovered in fences in the DeMilitarized Zone dividing the two countries, setting off an alert in the South for possible North Korean infiltrators.

Earlier last week, South Korea hunted - without success - for two North Korean submarines that were thought to have crossed into the South's waters.

Some see these actions as an attempt by the communist state to stir up South Korea. "They're trying to provoke South Koreans," says Hideshi Takesada, a professor at Japan's National Institute for Defense Studies. "Number one is North Korea may test South Korean capabilities to deal with these incidents. Second is if South Korea counters, North Korea may say 'oh, North Korea cannot talk with the South Korean government.'"

Clashes in the disputed waters of the Yellow Sea have happened before, especially during crab fishing season. In 1999 and 2002, skirmishes resulted in casualties on both sides.

The latest incident came the same day that South Korea took full responsibility for guarding its side of the DeMilitarized Zone. The United States withdrew its troops from the border, ending patrols there that date back to 1953, when the Korean War ended with an armistice, but no peace treaty.

Also Monday, North Korea accused the United States of trying to delay multi-party talks about the communist state's nuclear weapons programs.

Rodong Sinmun, the main newspaper in North Korea, says the Bush administration is seeking domestic political gain by saying Pyongyang could gain a lot by returning to the negotiating table.

A fourth round of talks in Beijing is on hold after North Korea refused to attend. The other parties - South Korea, the United States, China, Japan and Russia - have all urged North Korea to return to the table.

The newspaper blamed the United States for the stalled talks, saying Washington needed to offer new proposals for Pyongyang to give up its nuclear programs.