The United States and South Korea began on Sunday large-scale military exercises aimed at warning North Korea against any provocative and belligerent actions in the future, the Yonhap news agency reported.
The four-day naval and air combat drills, code-named Invincible Spirit, were originally scheduled for June 8-11, but they were postponed as the allies decided to wait for a UN resolution on the sinking of South Korea's Cheonan corvette.
The drills involve over 8,000 personnel, about 20 warships, including nuclear-powered Nimitz class aircraft carrier George Washington, and more than 200 combat aircraft.
North Korea earlier said the drills violate the country's sovereignty and security on the Korean Peninsula and Asia as a whole. Pyongyang threatened on Saturday it will start "a retaliatory sacred war" against the United States and its "lackeys."
"The army and people of North Korea will start a retaliatory sacred war of their own style based on nuclear deterrent any time necessary in order to counter the U.S. imperialists and the South Korean puppet forces deliberately pushing the situation to the brink of a war," the North's National Defense Commission said in a statement.
Washington and Seoul earlier said the drills were designed to improve coordination and interoperability between the allies and were not meant as a provocation.
Anti-submarine warfare operations have been scheduled as a key part of the joint exercises to prevent further attacks like the sinking of the Cheonan.
The 1,200-ton South Korean warship sank near the disputed Northern Limit Line in the Yellow Sea on March 26, causing the loss of 46 lives. South Korea says it has proof that North Korea fired a torpedo at the vessel from a submarine, although Pyongyang denies the attack.
The UN Security Council condemned the incident and stressed the need to prevent similar attacks in the region, but did not place the direct responsibility on North Korea.