The United States and South Korea are beginning a series of naval exercises off the Korean peninsula, in spite of threats from North Korea that it might retaliate.
The manoevres involve 8000 members of the US and South Korean forces, 20 ships and 200 planes.
North Korea says it will use its "nuclear deterrent" in response to the joint military exercise.
Defence officials quoted by the state-run Korean Central News Agency said Pyongyang was ready to launch a "retaliatory sacred war".
The US and South Korea aim to send what they have called a clear signal to the North, after the sinking of a South Korean warship in March, the BBC reports.
An international investigation said the ship was sunk by a North Korean torpedo, a claim strongly denied by Pyongyang.