The plot was foiled, it said, but gave no details on the fate of "Kim".
The denouncement of the USA and South Korea governments follows a visit from CIA Director Mike Pompeo to Seoul this week, The Washington Post reported.
US President Donald Trump has promised to "solve" North Korea and stop it developing nuclear weapons.
State media claims US and South Korean intelligence agencies were working together with an unidentified terrorist group to carry out an attack, possibly with a biochemical weapon at a military parade.
North Korean leader Kim Jon Un oversees a combat drill of the Korean People's Army in this undated photo released by North Korea's Central News Agency, Dec. 11, 2016.
"They hatched a plot of letting human scum Kim commit bomb terrorism targeting the supreme leadership during events at the Kumsusan Palace of the Sun and at a military parade and public procession after his return home", KCNA said.
The report claimed a North Korean citizen was involved in the plot, using "biochemical substances, including radioactive substance and nano poisonous substance" to target Kim. It said the plot was a "last-ditch effort" by United States "imperialists" and South Korea that had gone "beyond the limits".
June 2014: A North Korean working at a timber firm in the Khabarovsk territory of far-eastern Russian Federation is "corrupted and bribed" by CIA and South Korean intelligence agents.
The ministry refers to two payments to "Kim" of $US20,000 and a further two of $100,000 for "bribery" and obtaining equipment. Such an operation would be extremely hard to prepare and carry out successfully.
On Thursday, a think tank report from inside the US Congress said that the US' controversial THAAD missile defense system that has begun to be deployed in South Korea may not be capable of actually intercepting North Korean nukes directed around the Korean Peninsula and toward Japan.
North Korea warned this week that USA hostility had brought the region to the brink of nuclear war.
In July a year ago, North Korea presented to the media a man it alleged was a South Korean spy who tried to enter the North to kidnap children. It has described regular US-South Korea military exercises as plans to "decapitate" its leadership.
Analysts concur that a possible suspension of China's oil exports would have a major impact on North Korea, but they are more inclined to believe that China would not use its biggest punitive measure as leverage over North Korea-a total oil embargo-if Pyongyang is carrying out low-intensity provocations.
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