A South Korean navy patrol boat fired warning shots at North Korean patrol boats when they crossed a disputed sea border between the two countries on Sunday.
One of the North Korean patrol boats sailed around 1.6 miles into South Korean territory before warning shots were fired. A second entered less than an hour later, according to the Associated Press.
Both boats turned around after warning shots were fired. It's the first military encounter between the two nations following the sinking of a South Korean warship in March.
The cause of the sinking of the 1,200-ton patrol ship remains unclear; South Korean officials earlier raised the possibility of a torpedo having sunk the ship.
Although the North is widely suspected of being responsible for sinking the ship, the South Korean government has been careful not to directly accuse the North. But it has said that it will take stern action against anyone responsible for the disaster.