As North Korea ramps up its nuclear rhetoric, Julie Bishop has revealed Pyongyang has sent an unprecedented letter to the Australian government, urging it to turn away from US President Donald Trump.

Fairfax Media reports that the Foreign Minister confirmed the September 28 letter from North Korea’s Foreign Affairs Committee arrived via the country’s Jakarta embassy.

The letter lashes Mr Trump’s nuclear threats, saying his approach to foreign policy is “the height of American way of thinking that it is the best if the US is well-off at the expense of the whole world”.

“If Trump thinks that he would bring the [Democratic People’s Republic of Korea], a nuclear power, to its knees through nuclear war threat, it will be a big miscalculation and an expression of ignorance,” the letter reads.

The unusual approach comes as North Korea on Thursday threatened the US with nuclear annihilation over its military exercises on the Korean peninsula, vowing to unleash an “unimaginable strike at an unimaginable time”.

“The US is running amok by introducing under our nose the targets we have set as primary ones,” North Korean state news agency KCNA said in a statement.

“The U.S. should expect that it would face unimaginable strike at an unimaginable time.

“The rabid man in the White House … will first face the immense volley of nuclear fire if he hopes to settle (this) confrontation with nukes,” KCNA said.

North Korea’s letter to Australia said countries “loving independence, peace and justice” will be vigilant “against the heinous and reckless moves of the Trump administration trying to drive the world into a horrible nuclear disaster”.

Ms Bishop told Fairfax it was the first time Australia had received a letter from North Korea, which typically communicates through its state-run news agency KCNA, as it did on the weekend.

On Saturday, Pyongyang, via KCNA, again called out Australia for its outspokenness in calling for an end to its nuclear weapons program, warning it if it continues Australia “will not be able to avoid a disaster”.

At the time Ms Bishop, who was in South Korea with Defence Minister Marise Payne for talks with their South Korean counterparts, hit back and said Australia was not a “primary target”.

“North Korea’s threats only strengthen our resolve to find a peaceful solution to the rising tensions on the Korean Peninsula caused entirely by North Korea’s illegal, threatening and provocative behaviour,” Ms Bishop told reporters in Sydney on Sunday.

CIA Director Mike Pompeo on Friday morning (AEST) said North Korea was months away from perfecting its nuclear weapons capabilities.

“They are close enough now in their capabilities that from a US policy perspective we ought to behave as if we are on the cusp of them achieving” (their objective of being able to strike the United States), Mr Pompeo told a national security forum in Washington.

But he said there’s a difference between having the ability to fire a single nuclear missile and the capability of producing large amounts of fissile material and developing an arsenal of such weapons.

“When you’re now talking about months, our capacity to understand that at a detailed level is in some sense irrelevant,” he said.

– With AAP