The South Korean Navy fired warning shots Saturday night after two North Korean patrol boats crossed into South Korean waters, state media said.
The two North Korean patrol boats separately crossed a maritime border in the Yellow Sea. One retreated after receiving a warning communication from the South Korean Navy, and the other retreated after two rounds of warning shots were fired, Seoul's Joint Chiefs of Staff told the state-run Yonhap news agency.
Tensions between the two nations have run high since the mysterious March 26 sinking of a South Korean warship in the border area. Fifty-eight men escaped the sinking ship, but 46 of the 104 crew members died.
A team of South Korean military and civilian investigators tentatively concluded in April that an explosion at close range, and not a direct hit, sank the 1,200-ton ship. A U.S. military official also said he believes a North Korean torpedo attack was the most likely cause for the sinking.
The South Korean government has declined to explicitly name North Korea as the culprit in the attack. North Korean state media have also denied that the country has any involvement.
Saturday was the first time since the sinking of the ship that North Korean vessels have crossed the border, Yonhap reported. Both boats crossed the Northern Limit Line, the maritime border between the two states, and ventured about 1.2 miles (2 kilometers) into South Korean waters before turning back, Yonhap said.
North Korea has claimed the line, which covers rich crab fishing grounds, should be drawn farther south.
The border was the scene of fatal naval skirmishes in 1999 and 2002. The two Koreas also exchanged naval gunfire in 2004 and 2009.