The plot involved targeting Kim Jong-un at a public event, officials said
North Korea has accused US and South Korean agents of plotting to kill its Supreme Leader Kim Jong-un.
A North Korean referred to only as “Kim” was paid to carry out an attack with biochemical substances, the ministry of state security said.
It said the plot was foiled and the North would “ferret out and mercilessly destroy” US and South Korean agents. The US and Seoul have not commented.
The North’s claim comes amid continued high tension on the Korean peninsula.
US President Donald Trump has promised to “solve” North Korea and stop it developing nuclear weapons.
The ministry of state security statement, carried by state 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 discussion of the use of “biochemical substances including radioactive substance and nano poisonous substance” whose results would “appear after six or 12 months”, the statement said.
The ministry alleged that the North Korean man had been “corrupted and bribed” by South Korean intelligence services while he was working in Russia.
The ministry refers to two payments to “Kim” of $20,000 (£15,500) and a further two of $100,000 for “bribery” and obtaining equipment. It refers to another of $50,000 but it is unclear if this is additional.
It 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 the plot involved attacking a military parade or an event at the Kumsusan Palace of the Sun, the mausoleum of Kim Il-sung, the country’s founding leader.
Analysts say such an assassination operation would be extremely difficult to plan and carry out given the massive security around the supreme leader.
The ministry statement gave no details of how the alleged plan was uncovered or about the fate of “Kim”, who it described as “human scum”.
But it said an “anti-terrorist attack will be commenced from this moment to sweep away the intelligence and plot-breeding organisations of the US imperialists and the puppet clique,” referring to the South.
Earlier this year, Kim Jong-un’s half brother, Kim Jong-nam, was killed with the banned nerve agent VX at Kuala Lumpur international airport. South Korea and Malaysia said it was an assassination carried out by the North.
‘Major, major conflict’
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.
President Trump has warned of “major, major conflict” with North Korea.
But he also said this week he would be willing to meet Kim Jong-un, in the right circumstances. The White House quickly clarified that conditions were “clearly not there right now” and said the North should “end its provocative behaviour immediately”.