The South Korean navy has fired warning shots towards a North Korean fishing boat near their disputed sea border, the Defence Ministry said.

South Korea’s navy fired warning shots towards a North Korean fishing boat near their disputed sea border, the Defence Ministry said on Wednesday, the latest sign of tension on the divided peninsula ahead of next week’s Group of 20 summit in Seoul.

The ministry said the boat violated the western maritime boundary for about two hours before returning to North Korean waters early Wednesday. The maritime border, the scene of three deadly skirmishes between the Koreas, is a key flashpoint because the North does not recognize the U.N. line drawn at the end of the 1950—53 Korean War.

South Korea is bracing for any possible North Korean moves to sabotage next week’s Group of 20 summit of leading rich and developing countries.

The North has a track record of provocations during times when world attention is focused on Seoul.

In 1987, a year before Seoul hosted the Summer Olympics, North Korean agents planted a bomb on a South Korean plane, killing all 115 people on board. In 2002, when South Korea jointly hosted soccer’s World Cup along with Japan, a North Korean naval boat sank a South Korean patrol vessel near the sea border.

President Lee Myung—bak said in a televised news conference on Wednesday he did not believe that the North would take any such action, but that Seoul was ready for anything.

“The South Korean government is making thorough preparations against (any possible attacks) by North Korea and worldwide terrorist organizations,” Mr. Lee said.

His comments came a day after militants in southern Yemen blew up an oil pipeline operated by a state—owned South Korean company, Korea National Oil Corp. It was not clear whether al—Qaeda’s local offshoot was behind the attack, a Yemeni official said. Other anti—government militants are also active in southern Yemen.

Last week, South Korea’s military said that North Korea fired two rounds at a South Korean guard post in the Demilitarized Zone and South Korean troops immediately fired back. No injuries were reported, and the reason for the attack was unclear.

Tensions on the peninsula have been high since the mysterious sinking of a South Korean warship killed 46 sailors in March.

An international investigation concluded that a North Korean submarine fired a torpedo that sank the 1,200—ton Cheonan near the tense Korean sea border. North Korea flatly denied involvement at the time and warned that any punishment would mean war.

On Tuesday, North Korea issued a lengthy point—by—point denial. The 7,000—word statement by North Korea’s powerful National Defence Commission accused the South Korean—led investigation of fabricating data in its report. The U.S. also was part of the investigation.

North Korea disputed the probe’s conclusion that an aluminum torpedo sank the warship, saying all of its torpedoes are made of steel alloy. The statement said the North is willing to hand over parts of one of its torpedoes to South Korea for verification.

In Seoul, the Joint Chiefs of Staff dismissed North Korea’s latest denials as “nothing new.”

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