North Korea has said it is ready to pre-emptively attack and "liberate" South Korea in the latest attack on its arch-enemy's annual joint military drills with the United States.

In a statement reported by state media on Saturday, the General Staff of the North's Korean People's Army (KPA) said its frontline units were prepared to strike first if they saw signs that any US and South Korean troops involved in the drills were attempting to invade.

The KPA said it would counter the drills by the United States and South Korea with plans to "liberate the whole of South Korea including Seoul" and also that it was capable of executing "ultra-precision blitzkrieg" strikes.

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In response, South Korea's Joint Chiefs of Staff called for North Korea to stop what it called threats and "rash behavior" and warned that any provocation would result in the destruction of its leadership.

A pre-emptive large-scale strike by North Korea against the South is highly unlikely when that would almost certainly bring to an end the rule of leader Kim Jong Un given the likely military response of the US and South Korea.

South Korea and the US say the drills, which this year are described as the biggest ever, are defensive and routine.

The United States and South Korea remain technically at war with North Korea because the 1950-53 Korean War ended in a truce instead of a peace agreement.