The US and South Korea launched military manoeuvres involving three American aircraft carrier strike groups on Saturday in a massive show of force that drew the anger of rival North Korea.

The war exercises off the tense Korean Peninsula aim to strengthen the alliance's policy of "extended deterrence" against Pyongyang's nuclear and missile threats, South Korea's Joint Chiefs of Staff said in a statement.

The USS Ronald Reagan, USS Nimitz, and USS Theodore Roosevelt converged in the region for the four-day manoeuvres, also involving 11 American Aegis vessels and seven South Korean warships, including three destroyers.

South Korea's Yonhap news agency described the exercises as an "unprecedented show of force" off the peninsula. The last time such a convergence of military power occurred was in 2007 in waters off the US territory of Guam.

The military drills come as tensions remain sky-high as US President Donald Trump visits the region with the nuclear threat from North Korea dominating discussions.

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Speaking at the APEC summit in Da Nang, Vietnam, on Friday, Trump again called out North Korean leader Kim Jong-un and urged countries to "stand united" against the "greater and greater danger" posed by Pyongyang.

"The future of this region and its beautiful people must not be held hostage to a dictator's twisted fantasies of violent conquest and nuclear blackmail," said Trump, referring to Kim.

North Korea's foreign ministry issued a statement via the official Korean Central News Agency on Saturday describing Trump's trip as "a warmonger's visit for confrontation".

"It is also nothing but a business trip by a warmonger to enrich the monopolies of the US defence industry by milking the moneybags from its subordinate 'allies'," it added.

Pyongyang detonated its sixth and most powerful nuclear bomb in September after months of ballistic missile tests that riled Washington and its allies Japan and South Korea. The North says it needs nuclear weapons as a deterrent to prevent "invasion and plunder" by the US.

"The DPRK's possession of nuclear weapons was a righteous and inevitable choice to defend our national sovereignty and dignity and our people's rights to existence and development from increased nuclear threats and blackmail by the US and its hostile moves," the foreign ministry statement said, using the acronym of the country's official name, Democratic People's Republic of Korea.

Analysts say its only a matter of time before the North will gain the ability to fire a nuclear-tipped missile capable of reaching the United States.

Washington has said it will never accept a nuclear-armed North Korea.