The order for what it described as a "pre-emptive nuclear strike of justice" was made in a statement put out by the Supreme Command of the Korean People`s Army.

It came just days after leader Kim Jong-Un ordered the country`s nuclear arsenal to be placed on standby for use "at any moment," in response to tough new UN sanctions imposed over the North`s fourth nuclear test in January and last month`s long-range rocket launch.

Pyongyang has issued dire warnings of nuclear attack in the past, usually during periods of elevated military tensions of the Korean peninsula.

While the North is known to have a small stockpile of nuclear warheads, experts are divided about its ability to mount them on a working missile delivery system.

The national Defence Commission described the annual South Korea-US military exercises as "undisguised nuclear war drills" that threatened the North`s national sovereignty, and vowed an all-out offensive in response.

"The indiscriminate nuclear strike... will clearly show those keen on aggression and war, the military mettle of (North Korea)," said the statement published by the North`s official KCNA news agency.

Any strike would not just target operational theatres on the Korean peninsula, but also US bases on the mainland and in the rest of the Asia-Pacific region.

"If we push the buttons to annihilate the enemies even right now, all bases of provocations will be reduced to seas in flames and ashes in a moment," it added.

The annual exercises slated to begin Monday - called "Foal Eagle" and "Key Resolve" last for weeks and involve tens of thousands of US and South Korean troops.

Pyongyang has long condemned the drills as provocative rehearsals for invasion, while Seoul and Washington insist they are purely defensive in nature.

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