The military exercises, dubbed Invincible Spirit, are scheduled to run through July 28 in waters off South Korea to demonstrate the alliance's resolve. In addition to the 8,000 personnel involved, military officials say, the exercises will include 20 ships and submarines and about 200 aircraft.

The US Defense Department said the drills are in response to the sinking of the South Korean warship Cheonan and are intended to send a strong message to Pyongyang to stop "provocative and warlike acts."

The drills will include anti-sub infiltration exercises, said Cmdr. Jeff Davis of the US 7th Fleet.

"The anti-sub-infiltration exercise works like this: if a sub is coming in to attack a ship, the military finds it and prosecutes it," he said.