North Korea will seek the extradition of anyone involved in what it says was a CIA-backed plot to kill leader Kim Jong-un last month with a biochemical weapon, a top official says.

Han Song-ryol, the vice foreign minister, called a meeting of foreign diplomats on Thursday in Pyongyang to outline North Korea's allegation that the CIA and South Korea's intelligence agency bribed and coerced a North Korean man into joining in the assassination plot, which the North Korean Ministry of State Security has suggested was thwarted.

North Korea's permanent mission to the UN late on Thursday issued a statement calling the purported plot to kill Kim a "declaration of war".

It said the aim was to hurt "the mental mainstay that all the Korean people absolutely trust" and "eclipse the eternal sun" of the Democratic People's Republic of Korea (DPRK), the country's official name.

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The CIA and the White House declined to comment on the statements. The South Korean intelligence service said the charge was "groundless".

North Korean state media has been running stories about the plot since last week. The security ministry has vowed to "ferret out" anyone involved in the alleged plot, which it called "state-sponsored terrorism".

The permanent mission to the UN said the Ministry of State Security has declared that a "Korean-style anti-terrorist offensive will be commenced to mop up the intelligence and plot-breeding organisations of the US and South Korea".

Han took that a step further with the extradition statement.

"According to our law, the Central Public Prosecutor's Office of the DPRK will use all available methods to start to work to demand the handover of the criminals involved, so as to punish the organisers, conspirators, and followers of this terrible state-sponsored terrorism," he said.

North Korea claims the primary suspect is a man it has identified only by the ubiquitous surname "Kim". It says he is a North Korean resident of Pyongyang who worked for a time in the Russian Far East. State media said he was involved in the timber industry in Khabarovsk, which is one of the primary places North Koreans can go overseas to work.

The North further said a South Korean agent named Jo Ki-chol and a "secret agent" named Xu Guanghai, director general of the Qingdao NAZCA Trade Co Ltd, met Kim in Dandong, on North Korea's border with China, to give him communications equipment and cash. The North also said "a guy surnamed Han" taught Kim how to enlist accomplices.

"These terrorists plotted and planned in detail for the use of biochemical substances, including radioactive and poisonous substances, as the means of assassination," Vice Minister Han said, reading from a prepared statement.

"These biochemical substances were to be provided with the assistance of the CIA ... while the South Korean Intelligence Service was going to provide necessary support and funding for this attempt at assassination on our supreme leader."

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North Korea's UN mission said the organisers infiltrated "the terrorist" into the country with several pieces of satellite communications equipment so he could be updated "with the operational code of terrorism against the supreme leadership, various terrorist methods of using biochemical substances, ways of bribing and hiring the one who would actually carry out the terrorist act and ways of entering the venue of the event".

The statement said the organisers also gave him instructions to report on the "creed" of the person who would carry out the attack "and the state of his 'brainwashing", and to make sure the preparations were perfect, as there could be a war if it was revealed that South Korea's intelligence agency backed the operation.

In statements for foreign distribution, North Korea often refers to its leader Kim Jong-un without naming him, instead using the phrase "supreme leadership" or "supreme dignity".

The last time that Han appeared to brief foreign diplomats in Pyongyang was last December, to present North Korea's response to the latest round of UN sanctions after its September 2016 nuclear test.