SEOUL, South Korea » South Korea’s navy fired warning shots to chase away a North Korean fishing boat that crossed their disputed sea border early Wednesday, the Defense Ministry said, in the latest flareup of tension on the divided peninsula just days before the Group of 20 summit in Seoul.

The North Korean boat intruded on South Korean territory for about two hours before returning to North Korean waters early Wednesday, the ministry said. The fertile maritime border, the scene of three deadly skirmishes between the Koreas, is a key flashpoint because the North does not recognize the line drawn by the U.N. at the close of the 1950-53 Korean War.

The firing comes just days after North Korea shot two rounds at a South Korean guard post in the Demilitarized Zone, prompting return fire from South Korean troops, according to Seoul military officials.

South Korea is bracing for any possible North Korean moves to sabotage next week’s Group of 20 summit of world leaders. North Korea has a track record of provocations when world attention is focused on the rival South.

In 1987, a year before the Seoul 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 Wednesday that he does not believe Pyongyang would strike South Korea 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," Lee told reporters during a televised news conference.

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., according to company officials. It was not clear whether al-Qaida’s local offshoot was behind the attack, a Yemeni official said.

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 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 Defense Commission accused the South Korean-led investigation of fabricating data. 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."

Meanwhile, the U.S.-led United Nations Command on Wednesday returned the bodies of two North Korean soldiers that found in the river running through the heavily fortified inter-Korean border.

The village is jointly overseen by the U.N. Command and North Korea, an arrangement established in 1953 to supervise the cease-fire that ended the three-year war.