South Korea says that it has twice exchanged artillery fire with North Korea in western frontier waters. No casualties are reported.

Second round, artillery fired

South Korean military officials say a second round of artillery firing, presumably from North Korea, was heard about seven hours after an initial incident Wednesday.

A defense ministry spokesman in Seoul, who did not want to be named, says South Korea fired three warning shots into the Yellow Sea on Wednesday afternoon, an hour after North Korean shelling was heard close to the two countries' disputed sea boundary.

South Korean authorities say the North fired another round in the evening and the South's troops responded with three more shots.

The incidents occurred along a western maritime border, the Northern Limit Line. South Korea says it can not confirm that any shells fell into its waters, south of the line.

Shelling persumed from North

The shelling from the North apparently originated on Yongmae island, 11 kilometers north of the line.

A defense ministry spokesman says it is not sure if the North Koreans were conducting an exercise. Authorities have not determined whether the shots were fired southward deliberately or accidentally.

North Korea does not recognize the Northern Limit Line, contending that it should be re-drawn father south. South Korea regards the line, created by the United Nations Command in 1953, as the legal boundary.

South Korea's navy says it radioed a warning to the North 25 minutes after the first artillery rounds were heard and one shell landed close to the South's Yeonpyeong island.

South Korea's military then fired back from Yeonpyeong 35 minutes after it broadcast the warning.

South Korean military officials say there are no indications of unusual movements by the North Koreans and southern forces are maintaining a defensive posture.

There has been no immediate word from North Korea about Wednesday's incident. It is not unusual for any comments by the communist state on these types of incidents to emerge days later, if at all.

There have been three deadly naval clashes since 1999 near the disputed sea boundary.

Tensions remain high

Tensions have been especially high in the waters since North Korea shelled Yeonpyeong last November. Two South Korean marines and two civilians died and numerous houses were damaged in the barrage.

That incident came seven months after a South Korean navy ship exploded and sank in the Yellow Sea. An international investigation concluded that the Cheonan was hit by a North Korean torpedo. But North Korea denies any responsibility.

Seoul and Pyongyang have never signed a peace treaty following the 1953 armistice that halted the three-year Korean War.

torpedo: an explosive device that is set off in an oil well (or a gas well) to start or to increase the flow of oil (or gas) 魚(yú)雷

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（來(lái)源：VOA 編輯：實(shí)習(xí)生史莉萍）