SEOUL — The South Korean military fired three artillery rounds toward North Korea on Wednesday afternoon after hearing several explosions coming from the North Korean side of the disputed maritime border, according to a Ministry of National Defense spokesman.

The exchange took place near South Korea’s Yeonpyeong Island, which was shelled by North Korea last November, killing four South Koreans, including two civilians. The North Korean attack raised tensions between the two countries to their highest level in decades.

The defense ministry spokesman said three explosions believed to be artillery fire were heard coming from North Korea’s side of the Northern Limit Line, the sea border between the two Koreas. Around 1 p.m., South Korea responded by broadcasting a radio warning signal at 1:25 p.m., and then launching K-9 artillery rounds toward the North at 2 p.m. as a further warning, the spokesman said.

No rounds from either side crossed the sea border, he said. North Korea claims the line, which was established at the end of the Korean War, was unfairly drawn too far north.

“We are just investigating right now,” the spokesman said. “We don’t know if they were trying to hit South Korea.”

He said no further exchange of fire was expected on Wednesday, and South Korea has detected no movement of troops along the Northern border.

The skirmish is the latest in a series of incidents that have taken place in the last 18 months. In addition to the November attack, North Korea also sank a South Korean warship in March 2010 while it was on a routine patrol of waters along the border, killing 46 sailors on board.

South Korean government and military officials were roundly criticized in the media last year for responding with little more than rhetoric and economic sanctions following the attacks. In response, the South Korean military was restructured with the creation of the Northwest Islands Defense Command, that reports directly to the chairman of the South’s Joint Chiefs of Staff and tasked with defending its five westernmost islands from attacks by the North.

In a statement issued when the command was launched in June, South Korean President Lee Myung Bak said that when Yeonpyeong was attacked, “Our military did not rapidly and effectively face enemy provocations.”

“Only a group with “steadfast courage can ensure peace and security,” he said.

The South Korean military said it also has increased its manpower, firepower and equipment on and around the islands, and reportedly helicopter hangars will be built on Baengnyeong Island — just west of Yeonpyeong — to accommodate an undisclosed number of attack helicopters.

Former U.S. Forces Korea commander Gen. Walter Sharp said in June before his retirement that he worried about future North Korean provocations, especially as the impoverished North nears a self-imposed 2012 deadline of becoming a “strong and prosperous nation.” Numerous analysts and military and political officials have speculated that the Northern regime could try to demonstrate its power to its population by staging further military attacks.

“I do worry about how they will try to prove by 2012, or in 2012, that they are a great and prosperous nation,” he said.

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