North Korea threatened pre-emptive and "indiscriminate" nuclear strikes against South Korea and the United States on Monday

North Korea threatened pre-emptive and "indiscriminate" nuclear strikes against South Korea and the United States on Monday, as the two allies kicked off their annual, large-scale military exercises.

The drills always raise tensions on the divided Korean peninsula and the situation is particularly volatile this year, given the North's recent nuclear test and long-range rocket launch and its fury over tough UN sanctions imposed in response.

Participation in the joint exercises -- known as Key Resolve and Foal Eagle -- has been bumped up this year to involve 300,000 South Korean and around 17,000 US troops, as well as strategic US naval vessels and air force assets.

In a statement issued hours before the drills began, North Korea's powerful National Defense Commission said it was prepared for an "all-out" military counter-offensive.

Describing the exercises as "nuclear war drills" aimed at undermining North Korea's sovereignty, the statement said the Supreme Command of the Korean People's Army was ready to launch a "pre-emptive and offensive nuclear strike" in response.

The threat came just days after leader Kim Jong-Un ordered the country's nuclear arsenal to be placed on standby, in response to the sanctions resolution adopted last week by the UN Security Council.

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

The National Defense Commission said plans for what it called a "pre-emptive nuclear strike of justice" had been ratified by Kim Jong-Un.

The plans would come into operation in the event of "even the slightest military action" by the North's enemies, it said.

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

Targets would include operational American bases on the Korean peninsula and elsewhere in Asia, as well as the US mainland.