US and South Korean agents are plotting to kill North Korean leader Kim Jong-un, North Korean state media report.
A statement by the ministry of state security said a "terrorist group" backed by the CIA and South Korea's intelligence agency had entered the country in order to prepare an attack with a bio-chemical substance.
It said North Korea would find and "mercilessly destroy" the terrorists.
The claims come amid high tensions in the region.
US President Donald Trump has promised to "solve" North Korea and stop it developing nuclear weapons.
How was 'plot' to be carried out?
The statement, carried by North Korean news agency KCNA, said the CIA and South Korean intelligence services had "hatched a vicious plot to hurt the supreme leadership of the DPRK".
It did not mention Kim Jong-un by name, but he is widely referred to as the Supreme Leader.
The plot included the use of "biochemical substances including radioactive substance and nano poisonous substance" whose results would "appear after six or twelve months", the statement said.
The ministry alleges that a North Korean, which it identifies only by the surname "Kim", had been "corrupted and bribed" by South Korean intelligence services while he was working in Russia.
It lists several alleged payments made to him, amounting in total to nearly $300,000, and says on his return to Pyongyang he was instructed to provide detailed information about a frequently used event ground and to assess possible methods of attack.
The statement said an "anti-terrorist" attack would be "commenced from this moment," but gave no further details.
The KCNA news agency frequently carries bellicose rhetoric and threats against the US and South Korea,A war of words between the West and nuclear-armed North Korea has escalated in recent weeks, with Pyongyang threatening to carry out a sixth nuclear test.
On Saturday North Korea conducted its second failed ballistic missile test in two weeks.
The US has sent a warship to the region and installed a controversial anti-missile defence system in South Korea.
On Wednesday, US Secretary of State Rex Tillerson said Washington was preparing further sanctions for use "if it turns out North Korea's actions warrant additional sanctions", Reuters reported.
He also threatened secondary sanctions on companies doing illicit business with North Korea.