North Korea has said its military is ready to pre-emptively attack and "liberate" Seoul, in its latest outburst against the joint military drills by the United States and South Korea.

In a statement carried through state media, the North's Korean People's Army said its front-line units were prepared to strike first if they saw signs that American and South Korean troops involved in the annual drills were attempting to invade the North.

The KPA said it would counter the drills that it claims are aimed at advancing into Pyongyang with plans to "liberate the whole of South Korea including Seoul" and also that it is capable of executing "ultra-precision blitzkrieg" strikes against enemy targets.

At the start of the drills on Monday, the North warned of an indiscriminate "pre-emptive nuclear strike of justice" on Washington and Seoul.

North Korea has condemned the annual military drills staged by Seoul and Washington in South Korea, calling them preparations for an invasion.

The allies say the drills, which this year are described as the biggest ever and follow the North's recent nuclear test and long-range rocket launch, are defensive and routine.

South Korea's joint chiefs of staff called for North Korea to stop its threats and "rash behaviour" and warned that a provocation from Pyongyang would result in the destruction of its highest leadership.