North Korea threatened on Sunday to sink a US aircraft carrier cruising towards the tense Korean peninsula, and said it would strike Australia with nuclear weapons if it "blindly" followed its American ally.

The latest warnings from Pyongyang came as two Japanese navy ships joined the American carrier strike group for exercises in the western Pacific.

US President Donald Trump ordered the USS Carl Vinson to sail to waters off the Korean peninsula in response to rising tension over the North's nuclear and missile tests, and its threats to attack the United States and its regional allies.

US Vice President Mike Pence said on Saturday that it would arrive "within days" but gave no other details.

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North Korea's growing nuclear and missile threat is perhaps the most serious security challenge confronting Trump. He has vowed to prevent the North from being able to hit the US with a nuclear missile and has said "all options are on the table" - including a military strike.

North Korea, meanwhile, remained defiant.

"Our revolutionary forces are combat-ready to sink a US nuclear-powered aircraft carrier with a single strike," Rodong Sinmun, the newspaper of the North's ruling Workers' Party, said in a commentary on Sunday.

The paper likened the aircraft carrier to a "gross animal" and said a strike on it would be "an actual example to show our military's force".

The commentary was carried on page three of the newspaper, after a two-page feature about leader Kim Jong-un inspecting a pig farm.

Speaking during a visit to Greece, Chinese Foreign Minister Wang Yi said there were already enough shows of force and confrontation at present and appealed for calm.

"We need to issue peaceful and rational sounds," Wang said, according to a statement issued by China's foreign ministry.

"China is firmly supporting the denuclearisation of the area in the name of stability and peace," he told reporters in Athens.

Adding to the tension, North Korea detained a Korean American man in his 50s on Saturday, South Korea's Yonhap news agency reported, bringing the total number of US citizens held by Pyongyang to three.

The man, identified only by his surname Kim, had been in North Korea for a month to discuss relief activities, Yonhap said. He was arrested at Pyongyang International Airport on his way out of the country.

North Korea will mark the 85th anniversary of the foundation of its Korean People's Army on Tuesday. It has in the past marked important anniversaries with tests of its weapons.

Pyongyang has conducted five nuclear tests, two of them last year, and is working to develop nuclear-tipped missiles that can reach the US. It has also carried out a series of ballistic missile tests in defiance of United Nations sanctions.

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North Korea says its nuclear programme is for self-defence and has warned the United States of a nuclear attack in response to any aggression. It has also threatened to lay waste to South Korea and Japan.

On Sunday, it also added Australia to its list of potential targets.

A North Korean foreign ministry spokesperson accused Australia of following the US moves "blindly", according the Pyongyang's official Korean Central News Agency (KCNA).

"If Australia persists in following the US moves to isolate and stifle the DPRK [Democratic Peoples' Republic of Korea] and remains a shock brigade of the US master, this will be a suicidal act of coming within the range of the nuclear strike of the strategic force of the DPRK," the KCNA report said.

The threat came after remarks by Julie Bishop, Australian foreign minister, who said Australia sees Pyongyang's weapons programme as a threat.

Jim Mattis, US defence chief, said on Friday that North Korea's recent statements were provocative but had proved to be hollow in the past and should not be trusted. "We've all come to hear their words repeatedly. Their word has not proven honest," he said.

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Japan's show of naval force reflects growing concern that North Korea could strike it with nuclear or chemical warheads.

Some Japanese ruling party politicians are urging Prime Minister Shinzo Abe to acquire strike weapons that could hit North Korean missile forces before any imminent attack.

Japan's navy, which is mostly a destroyer fleet, is the second largest in Asia after China's.

The two Japanese warships, the Samidare and Ashigara, left western Japan on Friday to join the Carl Vinson and will "practise a variety of tactics" with the US strike group, the Japan Maritime Self-Defense Force said in a statement.

The Japanese force did not specify where the exercises were taking place, but by Sunday the destroyers could have reached an area 2,500km south of Japan, which would be east of the Philippines.

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From there, it could take three days to reach waters off the Korean peninsula. Japan's ships would accompany the Carl Vinson north at least into the East China Sea, a source with knowledge of the plan told Reuters news agency.

US and South Korean officials have been saying for weeks the North could soon stage another nuclear test, something the US, China and others have warned against.

South Korea has put its forces on heightened alert.

Last Thursday, Trump praised Chinese efforts to rein in "the menace of North Korea", after North Korean state media warned the US of a "super-mighty pre-emptive strike".