North Korea's official news agency described a US aircraft carrier patrolling off the Korean Peninsula as a "primary target" and said Washington should expect an "unimaginable" attack.

The latest threat on Thursday came amid massive joint US-South Korean naval exercises off the peninsula involving the American aircraft carrier the USS Ronald Reagan.

North Korea condemned Seoul and Washington's move to mobilise nuclear strategic assets near the volatile peninsula. Pyongyang has slammed the warship manoeuvres as a "rehearsal for war".

"The US is running amok by introducing under our nose the targets we have set as primary ones. The US should expect that it would face [an] unimaginable strike at an unimaginable time," said a statement carried by the official Korean Central News Agency.

Tensions have skyrocketed because of North Korea's recent nuclear and missile tests and exchanges of heated rhetoric between US President Donald Trump and North Korea's leadership.

The USS Ronald Reagan, a 100,000-tonne nuclear-powered aircraft carrier, patrolled in waters east of the Korean Peninsula on Thursday in a show of sea and air power.

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The US Navy's biggest warship in Asia, with a crew of 5,000 sailors, launched almost 90 F-18 Super Hornet sorties from its deck, in sight of South Korean islands.

It is conducting drills with the South Korean navy involving 40 warships deployed in a line stretching from the Yellow Sea west of the peninsula into the East Sea.

"The dangerous and aggressive behaviour by North Korea concerns everybody in the world," Rear Admiral Marc Dalton, commander of the USS Reagan's strike group, said as warplanes taxied on the flight deck above.

"We have made it clear with this exercise, and many others, that we are ready to defend the Republic of Korea."

North Korean leader Kim Jong-un has vowed to take the "highest-level" measures against the United States and Trump threatened to "totally destroy" North Korea if the US is forced to defend itself and its allies from a missile attack.

North Korea fired two intercontinental ballistic missiles in July and conducted its sixth and most powerful nuclear test last month.

The US military manoeuvres come ahead to Trump's visit to Japan and South Korea starting on November 5.