U.S. and South Korean troops have raised their alert levels to highest levels since 2006.
North Korea has renounced the 1953 truce with the South and threatened to strike any ships that tries to intercept its vessels.
The truce has been in place since the end of the Korean War.
The tensions began with North Korea's underground nuclear tests on Monday, followed by a series of short-range missles tests.
U.S. Secretary of State Hillary Clinton warned North Korea that there will be consequences.
"It has arbitraged the obligation is entered into thought the six party takes," she said. "And it continues to act in a provocative and belligerent manner toward its neighbors. There are consequences to such actions."
Clinton also said she is ready and willing to work with China and Russia to pressure North Korea.