South Korean Navy vessels berthed off the South Korea-controlled island of Yeonpyeong near the disputed waters of the Yellow Sea on December 22, 2010.

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The North Korean fishing boats retreated after the warning, the South says

The South Korean Navy issued warning messages before firing

Fatal naval clashes have taken place periodically between the two Koreas

Seoul, South Korea CNN  — 

The South Korean Navy fired warning shots to ward off North Korean fishing boats that were spotted south of the maritime border between the two countries, the South Korean Joint Chiefs of Staff said Friday.

The North Korean vessels crossed back over the border, known as the Northern Limit Line, following the warning shots, according to the Joint Chiefs of Staff.

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The South Korean Navy issued two warning messages to the North Korean boats before firing a few dozen shots, the Joint Chiefs of Staff said, adding that the alert level for the navy had been raised.

The were no immediate reports on the incident Friday by the North’s official Korean Central News Agency.

The two Koreas, which remain technically at war following their conflict in the early 1950s, have periodically clashed at sea over the years, with some waters still in dispute. The land border between the two nations remains heavily militarized.

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Fatal naval skirmishes took place near the inter-Korean maritime border in the Yellow Sea in 1999, 2002 and 2009.

And in 2010, Seoul accused Pyongyang of sinking one of its naval vessels near disputed waters off North Korea, killing 46 South Korean sailors. North Korea has repeatedly denied responsibility for causing the sinking.

South Korea is a key ally of the United States, with tens of thousands of U.S. troops stationed in the country.