South and North Korean patrol ships traded fire in the West Sea on Tuesday morning after a North Korean warship violated the Northern Limit Line, a de facto sea border.
There were no casualties on either side, Seoul's Joint Chiefs of Staff said.
Although each side only fired warning shots, it was the first time in nearly five years that the two navies had exchanged fire. The two sides exchanged fire some 8.8 kilometres away from each other.
"At around 9:50 a.m., a North Korean patrol ship violated the NLL and moved some 900 metres into South Korean waters. Our vessel communicated a warning to the North's ship and fired warning shots," a JCS official told reporters.
"The North then fired back, and our side also fired again. But neither side aimed their shots at the other side. Both just fired untargeted warning shots. We are currently strengthening our monitoring activities and maintain a robust readiness."
The South fired some 90 shots while the North is estimated to have fired scores of shots back.
"The North fired its machine guns, but their range was short and the shots landed several kilometers away from our ship. The North Korean vessel apparently fired the shots knowing that they would not reach the South Korean ship," the JCS official said.
The JCS presumed that the North crossed the NLL either to regulate the operations of North Korean fishing boats or to nullify the maritime border. Pyongyang has long disputed the legitimacy of the NLL, arguing that it was unilaterally drawn by the US-led UN Command after the end of the 1950-53 Korean War.
North Korean fishing boats frequently violate the NLL. On Sept 19, a fishing boat violated the NLL in the West Sea and returned to the North after South Korea's warning shots.
The exchange of fire came as the two sides have agreed to hold high-level talks later this month or early next month in the hopes of a thaw in cross-border relations. The two sides are working to determine the venue, date and agenda for the meeting, which would be the second of its kind since the first one was held in February.