The North Korean army is warning Washington that its military has been cleared to wage an attack using "smaller, lighter and diversified nuclear" weapons.
The threat from the unnamed army spokesman is the latest in a series of escalating warnings from North Korea.
It has railed for weeks against joint US and South Korean military exercises taking place in South Korea and has expressed anger over tightened sanctions for a February nuclear test.
The spokesman said in a statement carried by the Korean Central News Agency that troops have been authorised to counter US aggression with "powerful practical military counteractions".
US Defence Secretary Chuck Hagel is calling North Korea's development of nuclear weapons a "growing threat" to the US and its allies.
In a telephone call to Chinese Defence Minister Chang Wanquan, Mr Hagel cited North Korea's pursuit of nuclear weapons and ballistic missiles and said Washington and Beijing should continue to co-operate on those problems.
A Pentagon statement describing the phone call also disclosed that General Martin Dempsey, chairman of the Joint Chiefs of Staff, will visit China later this month. It would be his first trip to China as head of the Joint Chiefs.
Mr Hagel also invited the Chinese defence minister to visit the US this year.
Meanwhile North Korea yesterday barred South Korean workers from entering a jointly run factory park just over the heavily armed border in the North, officials in Seoul said.
The move came a day after Pyongyang announced it would restart its long-dormant plutonium reactor and increase production of nuclear weapons material.
The bid to bar South Koreans from entering the Kaesong factory park, the last remaining symbol of detente between the rivals, comes amid increasing hostility from Pyongyang.
It has said that the armistice ending the 1950s Korean War is void.
Seoul's Unification Ministry spokesman Kim Hyung-suk said Pyongyang is allowing South Koreans to return home from Kaesong, but that about 480 South Koreans who had planned to travel to the park yesterday were being refused entry.
He said North Korea cited recent political circumstances on the Korean Peninsula when they delivered their decision to block South Korean workers from entering Kaesong.
The two sides do not allow their citizens to travel to the other country without approval, but an exception has previously been made each day for the South Koreans working at Kaesong.
The Korean Peninsula is technically in a state of war because the Korean War ended in a truce not a peace treaty.
North Korea is angry about current South Korea-US military drills and new UN sanctions over its February 12 nuclear test, its third.
Dozens of South Korean firms run factories in the border town of Kaesong. Using North Korea's cheap, efficient labour, the Kaesong complex produced 470 million US dollars (£310 million) worth of goods last year.
Pyongyang threatened last week to shut down the park, which is run with mostly North Korean labour and South Korean know-how.
It expressed anger over South Korean media reports that said North Korea would not shut the park because it is a source of crucial hard currency for the impoverished country.
In 2009, North Korea closed its border gate in anger over US-South Korean military drills, leaving hundreds of South Korean workers stranded in Kaesong for several days. The park later resumed normal operations.