The military drills, code-named "Invincible Spirit," are to run Sunday through Wednesday with about 8,000 U.S. and South Korean troops, 20 ships and submarines and 200 aircraft. The Nimitz-class USS George Washington, with several thousand sailors and dozens of fighters aboard, was deployed from Japan.
The North routinely threatens attacks whenever South Korea and the U.S. hold joint military drills, which Pyongyang sees as a rehearsal for an invasion. The U.S. keeps 28,500 troops in South Korea and another 50,000 in Japan, but says it has no intention of invading the North.
Still, the North's latest rhetoric threatening "nuclear deterrence" and "sacred war" carries extra weight following the sinking of a South Korean warship that killed 46 sailors. Seoul and Washington say a North Korean torpedo was responsible for the March sinking of the Cheonan, considered the worst military attack on the South since the 1950-53 Korean War.
The American and South Korean defense chiefs announced earlier in the week they would stage the military drills to send a clear message to North Korea to stop its "aggressive" behavior.
The exercises will be the first in a series of U.S.-South Korean maneuvers to be conducted in the Sea of Japan off Korea's east coast and in the Yellow Sea closer to China's shores in international waters. The exercises also are the first to employ the F-22 stealth fighter which can evade North Korean air defenses in South Korea.
South Korea was closely monitoring North Korea's military, but no unusual activity had been observed Saturday, South Korea's Yonhap news agency reported.
North Korea, which denies any involvement in the sinking of the Cheonan warship, has warned the United States against attempting to punish it.
"The army and people of the DPRK will legitimately counter with their powerful nuclear deterrence the largest-ever nuclear war exercises to be staged by the U.S. and the South Korean puppet forces," North Korea's official news agency in Pyongyang quoted an unnamed government spokesman as saying. North Korea's official name is the Democratic People's Republic of Korea.
Though the impoverished North has a large conventional military and the capability to build nuclear weapons, it is not believed to have the technology needed to use nuclear devices as warheads.
Its rhetoric regarding using nuclear deterrence was seen by most as bluster, but its angry response to the maneuvers underscores the rising tensions in the region.
U.S. Secretary of State Hillary Rodham Clinton announced Wednesday, after visiting the Demilitarized Zone dividing the two Koreas, that the U.S. would slap new sanctions on the North to stifle its nuclear ambitions and punish it for the Cheonan sinking.
On Friday, the European Union said it, too, would consider new sanctions on North Korea.
The North's Foreign Ministry said Saturday that Pyongyang will further strengthen its nuclear deterrent and again mentioned "powerful physical measures" in response to the U.S. military provocations and sanctions.
In an apparent bow to China, the George Washington will participate in the exercise in the Sea of Japan, but there are no plans for it to enter the Yellow Sea for the subsequent exercises.
China, a traditional North Korean ally, has voiced concerns that military drills in the Yellow Sea could inflame tensions on the Korean Peninsula and also fears exercises too close to its own shores could breach Chinese security.
The George Washington had been expected to join in exercises off Korea sooner, but the Navy delayed those plans as the United Nations Security Council met to deliberate what action it should take over the Cheonan sinking.
The council eventually condemned the incident, but stopped short of naming North Korea as the perpetrator.