Reclusive North Korea has often issued threats to attack the South and the United States but has rarely turned them into action. Such hostile rhetoric is widely seen as a way to push its domestic and international political agenda.
A spokesman for the North's military warned the United States of "disastrous consequences" for moving a group of ships, including an aircraft carrier, into a South Korean port.
"In this connection, the units of all services and army corps level of the KPA received an emergency order from its supreme command to re-examine the operation plans already ratified by it and keep themselves fully ready to promptly launch operations anytime," the spokesman said, referring to the Korean People's Army (KPA).
"The U.S. will be wholly accountable for the unexpected horrible disaster to be met by its imperialist aggression forces' nuclear strike means," the spokesman said in a statement carried by the official KCNA news agency.
South Korea's Defense Minister Kim Kwan-jin said later on Tuesday there was no indication of unusual activity by the North's military.
Washington brushed off the North's warning.
