The annual exercises 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.
"As the joint military exercises to be staged by the enemies are regarded as the most undisguised nuclear war drills aimed to infringe upon the sovereignty of (North Korea), its military counteraction will be more pre-emptive and offensive nuclear strike to cope with them," it said, AFP reported.
The threat came just days after North Korea's 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.
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.
Participation in the joint drills -- known as Key Resolve and Foal Eagle -- has been bumped up this year to involve 300,000 South Korean and 15,000 US troops, as well as strategic US naval vessels and air force assets.
In a statement issued hours before the exercises began, North Korea's powerful National Defense Commission said it was prepared for an "all-out" military counter-offensive.
"As the joint military exercises to be staged by the enemies are regarded as the most undisguised nuclear war drills aimed to infringe upon the sovereignty of (North Korea), its military counteraction will be more pre-emptive and offensive nuclear strike to cope with them," it said.
Pyongyang has issued similar, dire warnings of nuclear attack in the past, usually during periods of elevated military tensions.