Tensions between North and South Korea have risen after they exchanged warning shots off the west coast of the peninsula on Monday.
Governments from both nations accused each other of breaching their maritime borders, in a sign the South may be taking a stricter line towards its rogue neighbour to the north.
A statement from the South's Joint Chiefs of Staff (JCS) said warnings had been broadcast and warning shots fired after a merchant vessel crossed the Northern Limit Line (NLL) at about 3:40am local time (6:40pm UK time).