South Korea launched its largest-ever anti-submarine exercise near the disputed sea border with North Korea, the defence ministry said, despite the North's threats of retaliation.
The South has warned the North it will not tolerate provocations during the five-day naval drill in the Yellow Sea.
The exercise is designed as a warning to the North following its alleged torpedo attack in March on a South Korean warship which killed 46 sailors.
Military officials say 29 ships including a submarine and destroyer, 50 fixed-wing aircraft and 4,500 army, navy, air force, marine and coastguard personnel are taking part.
A ministry spokesman said that underwater firing drills would take place close to Baengnyeong, the nearest island to the disputed maritime border with the North.
The warship went down just to the southwest of the island.