South Korea staged a naval exercise involving US surveillance aircraft on Tuesday, flexing its military muscles at a time of high tensions on the Korean Peninsula following the North's third nuclear test.
Day one of a six-day drill in the East Sea (Sea of Japan) involved South Korean warships and submarines, as well as surveillance planes from the United States, the defence ministry in Seoul said.
The exercises will practise operations to detect and track North Korean missiles and submarines, the ministry added.
"The drill is aimed at testing our readiness against threats from the enemy," a spokesman told AFP.
It is the latest in a series of military exercises, which have been staged alone by South Korea or jointly with the United States since the North launched a long-range rocket on December 12.
Earlier this month Seoul and Washington conducted a joint naval exercise with a US nuclear submarine off South Korea's east coast, followed by a joint airforce drill.
Pyongyang insists the December launch put a satellite into orbit for peaceful research, but critics said it amounted to a banned ballistic missile test that marked a major advance for the communist state's nuclear weapons programme.
Following the North's nuclear test on February 12, the South vowed to accelerate the development of longer-range ballistic missiles and new cruise missiles capable of a precision strike on members of Pyongyang's high command.
North Korea said the test -- widely condemned by the international community -- was a direct response to UN sanctions imposed on Pyongyang after last year's rocket launch.
Pyongyang is already under international sanctions for conducting two nuclear tests in 2006 and 2009, which both came after long-range rocket launches.
South Korea's outgoing President Lee Myung-Bak said Pyongyang was drawing closer to "a dead-end" by inviting isolation and sanctions from the international community.
"Though North Korea is congratulating itself on the success of the test, neither nuclear power nor missiles will protect them," he said in a farewell speech Tuesday.
Lee will leave office next week.