North Korean leader Kim Jong-un visited two military units near the maritime border with the South on its western coast, according to state media on Friday.
South Korea has accused Pyongyang of assassination plots, too, most recently in February following the murder of Kim Jong Nam, allegedly at the hands of North Korean agents.
North Korean officials said they were beginning an "anti-terrorist attack" that would "sweep away the intelligence and plot-breeding organisations of the United States imperialists and the puppet clique" after the alleged plot was uncovered.
"We will ferret out and mercilessly destroy to the last one the terrorists of the US CIA and South Korea's National Intelligence Service", the North Korean Ministry of State Security said in an English-language statement on Friday.
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"(North Korean leader) Kim Jong Un needs to maintain his lifestyle, so he will never start a war.
It is likely that any economic pressure will negatively affect China, which is North Korea's closest trading partner. The alleged plot was a "hideous crime" the security ministry said, and tantamount to "the declaration of a war".
Bruce Klingner, a former Central Intelligence Agency analyst now at the Heritage Foundation think tank, said it could be the first time North Korea had accused the allies of an attempted assassination.
The North Korean state media released little information about the alleged assassin, only that he was "human scum" who received about $750,000 from the USA and was working with a contact in South Korean intelligence.
The allegation did not explicitly say the method by which the individual would expose the North Korean leader to the poison, but claimed that "lethal results would appear after six or twelve months" and asserted that "only the CIA" could produce the substance that would be used.
China 'putting pressure' on North Korea
The North, defying mounting U.S. pressure, launched its latest missile test on Saturday, which South Korea said failed. Analysts say the KN-17 is a new Scud-type missile developed by North Korea.
With President Trump promising to stop North Korea developing nuclear weapons, these are also tense times, when Pyongyang could have been tempted to hit back at the US. In North Korean propaganda, South Korea remains a "puppet" of the United States.
The administration has said all options - including a military strikes - remain on the table.
The North Korean statement said the plan had "been put into the extremely serious phase of implementation". The White House quickly clarified that conditions were "clearly not there right now" and said the North should "end its provocative behaviour immediately".
Although a total oil embargo is unlikely, analysts see some kind of curtailment of Chinese exports to North Korea as probable.
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