North Korea says it is working on plans for a missile strike near the US Pacific territory of Guam, calling President Donald Trump's warning of "fire and fury" a "load of nonsense".

Key points: North Korea plans to land missiles 30-40km off Guam

North Korea plans to land missiles 30-40km off Guam US Defence Secretary says North's actions could "lead to end of regime"

US Defence Secretary says North's actions could "lead to end of regime" Donald Trump says US nuclear arsenal more powerful than ever

Pyongyang's state-run KCNA news agency issued an update on its strike plans after Mr Trump's comments that threats to the United States from Pyongyang would be met with "fire and fury".

Mr Trump's unexpected remarks prompted North Korea to say it was considering plans to fire four intermediate-range missiles to land 30-40 kilometres from Guam, home to about 163,000 people and a US military base that includes a submarine squadron, an air base and a Coast Guard group.

The army will complete its plans in mid-August, ready for North Korean leader Kim Jong-un's order, KCNA said on Wednesday, citing General Kim Rak Gyom, commander of the Strategic Force of the Korean People's Army.

Kim Jong-un will soon receive plans from the army for a strike on Guam. ( Reuters: KCNA )

The news agency said Mr Trump, "let out a load of nonsense about 'fire and fury'".

"Sound dialogue is not possible with such a guy bereft of reason, and only absolute force can work on him," it added.

North Korea said it was considering firing four intermediate-range missiles to land 30-40 kilometres from Guam.

Under the plan the Hwasong-12 rockets would cross the sky above Japan's Shimane, Hiroshima and Koichi prefectures, flying 3,356.7 kilometres for 1,065 seconds before landing in the ocean near Guam.

Japan could legally intercept a North Korean missile headed towards Guam if it posed an existential threat, Defence Minister Itsunori Onodera said in remarks reported by the Kyodo news service.

Mr Onodera told a parliamentary session Japan could exercise the right to collective self-defence and activate the Aegis destroyer ship-to-air missile defence system.

Earlier, experts said Japan did not currently have the capability to shoot down a missile flying over its territory headed for Guam.

US Defence Secretary Jim Mattis earlier issued a stark warning, telling Pyongyang the United States and its allies would win any arms race or conflict.

Sorry, this video has expired Philip Williams says Trump has matched the North Koreans in use of language (Photo: Reuters/KCNA)

"The DPRK should cease any consideration of actions that would lead to the end of its regime and the destruction of its people," General Mattis said in a statement, using the acronym for North Korea's official name, the Democratic People's Republic of Korea.

"The DPRK regime's actions will continue to be grossly overmatched by ours and would lose any arms race or conflict it initiates."

South Korea's military said North Korea would face a "stern and strong" response from Washington and Seoul if it acted on threats to fire missiles near Guam.

Joint Chiefs of Staff spokesman, Roh Jae-cheon, said South Korea was prepared to act immediately against any North Korean provocation, although the military had not spotted any unusual activity in the North.

The United States and South Korea remain technically still at war with North Korea after the 1950-53 Korean conflict ended with a truce, not a peace treaty.

Tension in the region has risen since North Korea carried out two nuclear bomb tests last year and two intercontinental ballistic missile tests in July.

Mr Trump has said he would not allow Pyongyang to develop a nuclear weapon capable of hitting the United States.

On Wednesday, Mr Trump followed up his "fire and fury" warning with a boast about US nuclear capabilities.

"My first order as President was to renovate and modernise our nuclear arsenal. It is now far stronger and more powerful than ever before," Mr Trump tweeted.

"Hopefully we will never have to use this power, but there will never be a time that we are not the most powerful nation in the world!"

Mr Trump's "fire and fury" remarks prompted warnings from US officials and analysts not to engage in rhetorical games with Pyongyang.

'Kim Jong-un doesn't understand diplomatic language'

Sorry, this video has expired Tillerson says Trump's tough talk aims to send message to North Korea

US Secretary of State Rex Tillerson, who was in Guam on a previously scheduled visit, played down the rhetoric, saying he did not believe there was an imminent threat from North Korea and "Americans should sleep well at night".

"The President … is sending a strong message to North Korea in language that Kim Jong-un would understand, because he doesn't seem to understand diplomatic language," Mr Tillerson said.

Earlier on Wednesday, Guam Governor Eddie Calvo dismissed the threat and said the island was prepared for "any eventuality" with strategically placed defences.

He said he had been in touch with the White House and there was no change in the threat level.

North Korea, pursuing missile and nuclear weapons programs in defiance of UN Security Council resolutions, accused Washington of devising a "preventive war" and said any plans to execute this would be met with an, "all-out war, wiping out all the strongholds of enemies, including the US mainland".

Washington previously warned it was ready to use force if needed to stop North Korea's ballistic missile and nuclear programs but that it preferred global diplomatic action, including sanctions.

The UN Security Council unanimously imposed new sanctions on North Korea on Saturday.

A senior administration official who deals with the Korea issue said the "fire and fury" comment was "unplanned and spontaneous".

But White House spokeswoman Sarah Sanders told reporters national security officials had been aware of the tone of Mr Trump's message before he gave it.

The US Pacific territory of Guam is located in the Pacific Ocean. ( ABC News )

Reuters