The US and South Korea have commenced a major anti-submarine drill, to tackle provocations by North Korean submarines.
The three-day exercise is being carried out off the southern island of Jeju, and comprises more than ten vessels, including a South Korean Aegis destroyer.
The drill also includes submarines, surveillance planes, and helicopters equipped with air-to-ground missiles.
A spokesman for South Korea defence ministry said: "This is the largest joint anti-submarine exercise the allies have ever staged, in terms of its scale and the number of vessels involved."
The exercise comes after North Korea has tested a submarine-launched ballistic missile (SLBM) last month in line with efforts to strengthen its defence capabilities.
A South Korean Navy official stated: "The drill is aimed at deterring submarine-related provocations and beefing up the Navy's capacity to protect a southern maritime pathway, a key factor in maintaining operational logistics during war."
South Korean artillery units, naval ships and jet fighters have recently carried out a live-fire drill off the east coast of Goseong county.
In May, North Korea conducted a third test of SLBM from an underwater test platform near the country's coastal city of Sinpo.
In March, US Navy's littoral combat ship (LCS) USS Fort Worth completed a joint training exercise in South Korea.