AS US President Donald Trump and North Korean leader Kim Jong-un trade barbs over the threat of war, one state is already preparing for the worst possible outcome.

Foreign Policy reports Californian officials are urging local agencies to shore up their nuclear attack response plans. Last month, the Los Angeles Joint Regional Intelligence Center issued a report titled Nuclear Attack Response Considerations warning a nuclear strike on Southern California would be “catastrophic”.

It comes as Pyongyang accused Trump of declaring war on the hermit kingdom after the President tweeted: “Just heard Foreign Minister of North Korea speak at U.N. If he echoes thoughts of Little Rocket Man, they won’t be around much longer!”

The new report, reportedly obtained by Foreign Policy, cited North Korea’s intercontinental ballistic missile test in late July which could, in theory, reach the West Coast of the US.

It also referenced the rogue nation’s propaganda videos that showed the ruins of San Francisco and Washington.

“The consequences of a nuclear attack in Southern California would be catastrophic,” the report stated.

“Nonetheless, government entities and first responders are expected to remain operational to preserve human life, maintain order, and aid in the recovery process.”

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The 16-page report was circulated to all LA-based first responders including all local, state and federal agencies such as the Department of Homeland Security.

The document also explored the aftermath of a nuclear blast including the effects of radiation, the possibility of electromagnetic pulse disabling communications and the devastation from the initial blast on human life and infrastructure, Foreign Policy reported.

Citing figures from Rand Corp, a US global policy think tank, the report claimed a nuclear blast at the Long Beach Port could cause more than $1 trillion (AU$1.2 trillion) in damage and significant loss of life.

In another section called “radiation protection basics”, the report offers steps on what to do during a nuclear attack.

“Lie face down and place hands under the body to protect exposed skin,” it recommends. “Remain flat until the heat and shock waves have passed.”

It also warned government authorities would likely be of little help in the immediate aftermath of the blast, saying there will be “no significant federal assistance at the scene for 24-72 hours after the attack”.

The public would need to evacuate but with “limited understanding of radiation risks, they will experience high anxiety and may be non-compliant.”

It also flagged challenges with contamination spread by pets and through clothing among the myriad of public health and logistical coordination issues emergency responders will face.

The idea behind the unclassified report was to share planning and guidance with as wide a distribution network as possible, Foreign Policy reported citing two officials involved in responding to a nuclear strike.