The US and South Korea have embarked on their annual two-week joint military exercise as rival North Korea issued a stern warning that a "merciless pre-emptive strike will start anytime chosen by us".

More than 50,000 South Korean and 30,000 American personnel have been mobilised for the mostly computer-simulated drill, codenamed Ulchi Freedom Guardian (UFG).

The exercise is aimed at strengthening the coordinated defensive measures by both countries against increasing threats from Pyongyang.

International monitors from Switzerland and Sweden will be observing the exercise.

For the first time, Washington and Seoul intend to come up with a tailor-made deterrence strategy against North Korea's nuclear weapons or weapons of mass destruction.

Although such drills are often criticised by North Korea, its reaction this year appears to be harsher.

A statement from North Korea's General Staff of the Korean People's Army, carried by the state-run mouthpiece Korean Central News Agency (KCNA, read: "Now that the US imperialists and the south Korean puppet forces declared a war against the DPRK by announcing their plan to apply the "tailored deterrence strategy" to an actual war, we re-clarify that the Korean-style most powerful and advanced merciless pre-emptive strike will start any time chosen by us."

"If we strike the US imperialists and the South Korean puppet forces, they should be hit hard without any excuse and if we are determined, large and small bases for aggression will be reduced to a sea in flames and ashes."

North Korea's foreign ministry has also issued a similar statement condemning the exercise.

South Korean and the US forces are on heightened alert for the drill in the backdrop of the fresh North Korean belligerence.