North Korea has warned Australia to distance itself from the Trump US administration or face the consequences.
Australian Prime Minister Malcolm Turnbull has told the 3AW radio station that the one-page letter was "basically a rant" and had been sent to several other nations around the world.
In the communication, North Korean dictator Kim Jong-un insisted the Americans would not destroy his republic, as threatened, and would ultimately be responsible for a "horrible nuclear disaster".
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The North Koreans have recently escalated their nuclear testing, amid speculation they have developed missiles that could reach as far as New Zealand.
Tensions have heightened between Kim Jong-un and US President Donald Trump, with the latter mockingly referring to his counterpart as "Rocket Man".
On Saturday, the dictator warned that Australia would "not be able to avoid a disaster", if it aligned with the Americans' policies against Kim Jong-un.
But while Australian Foreign Minister Julie Bishop described the letter as "unprecedented", Mr Turnbull insisted it was a sign that global pressure on North Korea was taking its toll and was hopeful of further international engagement.