The news was announced in a statement by the National Defense Commission of North Korea and published in the state-run Korean Central News Agency.
"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 the DPRK, its military counteraction will be more preemptive and offensive nuclear strike to cope with them," the statement read.
The United States responded with a call for caution.
"We urge North Korea to refrain from provocative actions and statements that aggravate tensions and instead focus on fulfilling its international obligations and commitments," a senior administration official said Monday. "We are closely monitoring the situation on the Korean Peninsula in coordination with our Republic of Korea allies."
North Korea's bellicose words are typical around the time of annual military exercises, according to CNN's Paula Hancocks.
"They (North Korea) have threatened this before, and these kinds of threats are to be expected this time of year," she said.
But Hancocks noted that tensions this year are even higher than normal after recent action at the United Nations.
The Security Council voted last week
to impose an array of sanctions against North Korea because of that nation's recent nuclear test and missile launch, both of which defied international sanctions. The resolution that brought about the sanctions aims to cripple the economic factors that fuel the North's nuclear and ballistic missile programs.
The North Korean news agency has blasted the sanctions as "unprecedented and gangster-like."
Discussions about new sanctions started after North Korea claimed to have successfully tested a hydrogen bomb in January, its fourth nuclear test.
Then, in February, Pyongyang said it had successfully launched an Earth satellite into orbit via the long-range Kwangmyongsong carrier rocket.
About 17,000 U.S. forces will participate in the joint military exercises with South Korea, according to United States Forces Korea.
The two exercises, "Key Resolve" and "Foal Eagle," will run until April 30. "Foal Eagle" will involve ground, air, naval and special operations forces from both militaries, USFA said.