The US and South Korea on Sunday began a major military exercise in the Sea of Japan, ignoring threats from North Korea to launch a nuclear attack in response to US "aggression" if the drills went ahead.
Operation Invincible Spirit began amid heightened tensions on the Korean peninsula, four months after a torpedo sank a South Korean navy ship, killing 46 sailors.
International investigators said the torpedo had been fired from a North Korean submarine; Pyongyang, however, denies any involvement.
The regime has resisted calls to apologise for the sinking and vowed to retaliate militarily against any attempts to punish it.
The four days of naval and air drills in waters South Korea refers to as the East Sea will offer the north a timely reminder of the formidable firepower it would confront in any conflict with the US and its ally.
The operation involves 8,000 US and South Korean troops, 200 aircraft and 20 vessels, including three destroyers, led by the nuclear-powered aircraft carrier the USS George Washington.
US military officials said the exercises would send a "clear message" to the north to end its provocative behaviour.