For the fourth time this week, North Korean fishing boats have reportedly crossed a disputed border into South Korean waters, testing already strained relations over Pyongyang's nuclear programs. The latest incident comes as a group of U.S. congressmen are to hold meetings in Pyongyang to convince the communist state to give up its nuclear ambitions.

South Korean military officials say seven North Korean fishing boats crossed into South Korean waters off the western sea border on Friday. Seoul says it is the fourth such violation this week.

South Korea had already sent a protest letter through its Red Cross to Pyongyang on Wednesday.

A spokesman for South Korea's Joint Chiefs of Staff says the latest incursion lasted an hour and the boats only ventured about one-and-a-half kilometers south of the tense border. The spokesman says the North Korean boats, apparently catching lucrative crabs, retreated after warning messages from South Korean naval vessels.

The two Korean navies clashed in deadly battles in those same waters in 1999 and, again, last year.

North Korea has long claimed a boundary farther south than the western sea border demarcated by the United Nations at the end of the Korean War in 1953.

Meanwhile North Korea on Thursday accused the South of sending warships across the maritime boundary, warning South Korea that further moves could lead to "irrevocable serious consequences."

Tensions have been heightened on the peninsula since October when the United States said Pyongyang had admitted it was pursuing a nuclear weapons program in violation of international agreements.

In an effort to relieve some of the tension, a bipartisan delegation of U.S. congressmen has scheduled meetings in Pyongyang.

Representative Curt Weldon, a Republican from Pennsylvania speaking before the group's departure, said the politicians intend to tell North Korea it can expect economic aid and trade from the United States if it completely halts its nuclear programs.

The congressmen are not carrying any message from President Bush nor has the trip been sanctioned by the White House.

Congressman Weldon's office says the invitation from North Korea came after a year of negotiating that included assistance from U.N. Secretary General Kofi Annan and U.S. Secretary of State Colin Powell.