The North Korean Ministry of State Security described the plot as tantamount to "the declaration of a war."
North Korea has accused the US and South Korea of plotting to assassinate Kim Jong-Un.
Pyongyang claims the CIA and Seoul's Intelligence Services had "hatched a vicious plot" to kill the secretive state's young leader as he attended ceremonial events.
The Ministry of State Security alleges the agencies "ideologically corrupted and bribed a North Korean citizen surnnamed Kim" to orchestrate the attack - paying him tens of thousands of dollars.
It added that the CIA had chosen to use "biochemical substances" for the assassination - and described the plot as tantamount to "the declaration of a war."
The statement said: "We will ferret out and mercilessly destroy to the last one the terrorists of the US CIA and the puppet Intelligence Services of South Korea.
"This heinous crime, which was recently uncovered and smashed in the Democratic People's Republic of Korea, is a kind of terrorism against not only the DPRK but the justice and conscience of humankind."
No explanation was offered as to how the plot was foiled, or what happened to the North Korean citizen who was allegedly involved.
But the statement, which described the suspect as "human scum," warned: "Criminals going hell-bent to realise such a pipe dream cannot survive on this land even a moment."
The allegations came after the US House of Representatives voted overwhelmingly in favour of imposing tighter sanctions on North Korea by targeting its shipping industry.
Those in favour of the legislation said it is designed to send a strong message to Pyongyang that the escalation of its nuclear programme will not be tolerated.
The Senate would need to approve the sanctions before they can be sent to the White House for President Trump to sign into law.
It is likely that any economic pressure will negatively affect China, which is North Korea's closest trading partner. This is because the American sanctions would penalise companies that trade with Pyongyang.
China's foreign ministry has asked for all sides to exercise restraint and to avoid taking measures which could make the tense situation on the Korean Peninsula worse.