Seoul (AFP) - North Korea threatened "indiscriminate" nuclear strikes against South Korea and the US mainland, as the two allies prepared to kick off large-scale joint military drills on Monday.

The threat to carry out what it described as a "pre-emptive nuclear strike of justice" was made in a statement by the North's powerful National Defence Commission, citing the Supreme Command of the Korean People's Army (KPA).

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 on the divided 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 to "even the slightest military action."

"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.

Under a military plan ratified by the North Korean leadership, 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.

- 'Flames and ashes" -

"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.

Despite a pair of successful long-range rocket launches, North Korea is largely believed to be years away from developing a genuine inter-continental ballistic missile (ICBM) capable of reaching the US mainland.

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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.

Tensions have surged on the Korean peninsula since the North's nuclear test on January 6 and February's rocket launch, which was seen as a disguised ballistic missile test.

A UN Security Council resolution adopted last week laid out the toughest sanctions imposed on Pyongyang to date over its nuclear weapons programme and will, if implemented effectively, apply significant economic pressure on Kim's regime.

It breaks new ground by targeting specific sectors key to the North Korean economy and seeking to undermine the North's use of, and access to, international transport systems.

Pyongyang has rejected the sanctions as "unfair, illicit and immoral" and vowed to keep building its nuclear arsenal.

The National Defence Commission said the US and its allies had failed to realise how the "outrageous" sanctions made "this land boil like a crucible of battle."