South Korean navy ships fired dozens of warning shots after North Korean (search) fishing boats crossed a disputed maritime border Sunday, South Korea's Defense Ministry said, in the sixth day of confrontations at sea in a week.

South Korea's Joint Chiefs of Staff said eight North Korean fishing boats had sailed into the southern side of the sea border off the Korean Peninsula's west coast.

The Defense Ministry said naval vessels broadcast warnings to the North Korean boats, which ignored them. Naval ships then fired 34 shots into the sea, and the North's fishing boats turned back, it said.

The ministry said it sent a strong message to North Korea to come up with "responsible measures" that would prevent further incursions.

"We make it clear that North Korea will be held completely responsible for any incidents that arise from North Korean boats violating the Northern Limit Line (search) along the west border," the ministry said in a statement.

The maritime border between the two Koreas is not clearly marked and was the source of tension and naval clashes in 1999 and 2002. North Korean fishing boats occasionally cross over into South Korean waters during the crab catching season, which is in full swing in June.

South Korea is studying whether the repeated violations over the past week were intentional, a military spokesman said.

The confrontations came amid a crisis over the North's nuclear programs. Washington has been trying to muster international pressure on the North to abandon its nuclear ambitions since October, when U.S. officials said North Korea admitted running a secret nuclear program in violation of a 1994 pact.

In St. Petersburg on Sunday, President Bush and Russian President Vladimir Putin urged North Korea and Iran to halt development of nuclear weapons.

Both the United States and Russia are "determined to meet the threat posed by weapons of mass destruction," Bush said at a joint news conference.

"We strongly urge North Korea to visibly, verifiably and irreversibly dismantle its nuclear weapons program," he said.

Also Sunday, Deputy Defense Secretary Paul Wolfowitz (search) arrived in South Korea after calling for international economic pressure to persuade North Korea to drop its nuclear weapons program.

At a regional security conference in Singapore on Saturday, Wolfowitz urged Asian nations to help Washington end the nuclear standoff with North Korea peacefully by putting economic pressure on Pyongyang.

North Korea suffers chronic food shortages and has depended on outside help since the mid-1990s to feed its 22 million people, and some in South Korea oppose sanctions against the North.

On Thursday, North Korea accused South Korean navy ships of repeatedly violating its territorial waters off the western coast and warned of "irrevocable serious consequences."

South Korea's Defense Ministry rejected the accusations.

The territorial dispute dates back to the end of the 1950-53 Korean War, with North Korea never recognizing a sea border drawn by the U.S.-led U.N. Command at the end of the conflict.

The Koreas were divided in 1945.