The U.S. and South Korea have started their largest-ever annual joint military exercises amid heightened tensions on the Korean Peninsula — and North Korean leader Kim Jong Un has threatened to respond to the exercises with a nuclear offensive.

The U.S. says the Korean People's Army in North Korea was informed about the drills and their dates by the United Nations Command. The maneuvers include a computer simulation of military attacks, as well as maneuvers in the field.

From Seoul, NPR's Elise Hu reports that:

"17,000 American troops are taking part in the joint military drills. 300,000 South Korean soldiers are involved. The exercises — called Key Resolve and Foal Eagle — happen every March. This year they come after neighboring North Korea conducted its fourth nuclear test and a long-range missile test."North Korea has been sensitive to these war games in the past. They have responded by firing projectiles into nearby seas. This year in advance of the drills, North Korean leader Kim Jong Un ordered his nuclear warheads to be ready. The joint exercises will continue through the end of April."

Responding to the start of the exercises, Kim threatened Monday to unleash a "preemptive nuclear strike of justice."

Ahead of the exercises, a three-vessel expeditionary strike group that includes the flagship amphibious assault ship USS Bonhomme Richard arrived at Busan, South Korea, and other ports last week. Those ships are carrying some 4,000 sailors and Marines, the Navy says.

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