North Korea said on Tuesday its military would be put on high alert and be ready to launch operations, stepping up tension after weeks of rhetoric directed against the United States and South Korea, who it accuses of instigating hostility.

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 means to perpetuate its domestic and international political agenda.

In the latest outburst, 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.

(Read more: Satellite image suggests North Korea has restarted Yongbyon nuclear reactor: US group)

"In this connection, the units of all services and army corps level of the KPA received an emergency order from its supreme command to reexamine the operation plans already ratified by it and keep themselves fully ready to promptly launch operations any time," 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.

