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

The General Staff of the North's Korean People's Army said Saturday in a statement that its frontline units are prepared to strike first if they see signs that American and South Korean troops involved in the drills were attempting to invade the North.

The statement comes after the North warned of pre-emptive nuclear strikes at the start of the drills on Monday.

North Korea has condemned the annual military drills staged by Seoul and Washington, calling them preparations for an invasion. The allies say the drills, which this year are described as the biggest ever, are defensive and routine.