SEOUL (Reuters) - 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.

"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.

In March, the North declared it was no longer bound by the armistice that ended fighting in the 1950-53 Korean War signed with the United States and China, threatening to use nuclear weapons to attack U.S. and South Korean territories.

The North has defied international warnings not to build nuclear and long-range missiles and is believed to have enough fissile material to build up to 10 nuclear bombs.

Most intelligence analysis says it has yet to master the technology to deploy such weapons.

The United States, which has 28,500 troops stationed in the South, regularly engages in drills with its ally, and has said the aircraft carrier USS George Washington was leading a group of ships to visit South Korea in a routine port call.

South Korea's Defense Ministry said on Monday the ships were taking part in a routine maritime search and rescue exercise and said any criticism by North Korea was "wrong".

The impoverished North's large but ageing conventional military is considered unfit to fight an extended modern battle but it staged surprise attacks against the South in 2010 that killed 50 people in aggression unprecedented since the war.

An attempt at dialogue in August led to the reopening of a jointly run factory park that was shut amid high tensions in April. However, talks have since hit a stalemate.

(Reporting by Jack Kim; Editing by Paul Tait)