The United States and South Korea began a major naval exercise Sunday, despite threats from North Korea that the drills could lead to war.
About 8,000 U.S. and South Korean troops are participating in the four-day-long exercise "Invincible Spirit" in the Sea of Japan.
Twenty warships are taking part, including the nuclear-powered aircraft carrier USS George Washington. F-22 Raptors are among some 200 aircraft involved in the exercise. The stealth fighters are the most advanced planes in the U.S. Air Force.
On Saturday, North Korea's National Defense Commission threatened to wage what it called a "sacred war" against the United States and South Korea if the two nations went ahead with the maneuvers.
U.S. and South Korean military leaders say the drills are a significant show of force intended to deter North Korea from further aggression.
The exercise and the North's threat follow the March sinking of a South Korean warship that killed 46 sailors. An international investigation concluded the vessel had been torpedoed by a North Korean submarine. Pyongyang has denied involvement.
The exercise was moved from the Yellow Sea off South Korea's west coast after strong objections from China. However, South Korean Rear Admiral Kim Kyung-shi denied that pressure from Beijing was the main reason the war game was shifted.
U.S. Secretary of State Hillary Clinton recently said North Korea is an "isolated and belligerent" regime engaged in a "campaign of provocative, dangerous behavior."
Clinton announced new economic sanctions against Pyongyang last week. North Korea's Foreign Ministry said Saturday those sanctions would be met with "strong physical actions."
North Korean spokesman Ri Tong Il described the joint military exercise as a threat to his nation's sovereignty.
Some information for this report was provided by AP and AFP.