The US and South Korean militaries kicked off large exercises today to underscore deterrence against North Korea after accusing the reclusive communist state of sinking a warship.
Pyongyang warned that the drill had pitched the peninsula onto the brink of war.
US naval vessels, including the nuclear-powered aircraft carrier USS George Washington, began the drills by setting off from South Korean ports where they had called last week in a show of force timed with a high-level meeting between the two allies.
North Korea drove tensions to new heights after a team of investigators, led by South Korea's military, accused it of firing a torpedo in March to sink the corvette Cheonan, killing 46 men.
The United States announced new sanctions on the North last week, freezing the assets of Pyongyang's leaders it said were earned through illcit activities and cutting off the flow of cash to them. The moves would also ban travel by some individuals.
China had objected to the drills.
Beijing criticized the introduction of large-scale military equipment into the Yellow Sea off the peninsula's west coast, prompting a move of the bulk of the exercises to areas off the east coast.
Yesterday, the North's powerful National Defense Commission vowed to launch a "sacred war" against the United States and South Korea at "any time necessary," in response to the drills, denounced as "reckless."
The drills involve more than 200 aircraft, including the F-22 Raptor fighter, and three destroyers, including the USS John S. McCain, part of the 97,000-tonne USS George Washington's strike group.
Four Japanese military officers will be on board the carrier to observe the drills.
Pyongyang has routinely been shrill in voicing its anger in the past when the allies conducted exercises.
But US officials say further provocations are possible in coming months, especially as the North tries to build political momentum for the succession to leader Kim Jong-il, expected to hand power to his youngest son.
North Korea has called for the resumption of six-party nuclear disarmament talks that it had boycotted since late 2008, a move analysts said was an attempt to put the Cheonan incident behind it and win lucrative aid through negotiations with the South, the United States, Japan, Russia and China.
On Saturday, the North's foreign ministry said it was ready for dialogue but vowed to respond by force if it had to.
"We are not the one who would be surprised by military threats or sanctions," a ministry spokesman said.