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.
"The murderous demons" of the NIS who conspired with the Central Intelligence Agency "corrupted and bribed a DPRK citizen surnamed Kim, the then worker of the timber industrial branch in the Khabarovsk Territory of Russian Federation in June 2014, and turned him into a terrorist full of repugnance and revenge against the supreme leadership of the DPRK", it said.
The statement, carried by North Korea's official news agency and read on state TV, didn't describe how the alleged plot was broken up or give the full name of the North Korean suspect, identifying him only by his surname, Kim, and didn't say whether anyone else was in custody.
The North Korean claim on Friday said Pyongyang will be launching its own intelligence operations to counter USA and South Korean spying and assassination plots against Mr. Kim.
The reclusive state, already highly sensitive to insults aimed at its "supreme leader", said it would strike back at operatives of the CIA and Seoul's National Intelligence Service.
A "terrorist group" supported by the Central Intelligence Agency and the South crossed the border before trying to kill the North Korean dictator, the nation claimed.
The CIA told its agent Kim it had access to radioactive and "nano poisonous" substances whose lethal results would appear only after six to 12 months, the statement said.
North Korea's Ministry of State Security called the supposed plot a "last-ditch effort" that had gone "beyond the limits".More news: United Kingdom could strike in Syria without MP vote
Pyongyang presented extensive details but offered no concrete proof to back its accusations of the plot, which it said could never have succeeded.
President Trump has warned of "major, major conflict" with North Korea.
The half-brother of Kim Jong Un was poisoned with the VX nerve agent while in Kuala Lumpur airport and died en route to a hospital. There was no news of how Kim smuggled the device back into North Korea or what had become of him.
The statement came hours after the US House of Representatives in Washington voted to broaden US sanctions against the North.
Then, in April, reports suggested that China's patience with North Korea was wearing thin as Pyongyang continued to conduct nuclear missile tests, and that Beijing was considering a suspension of its crude oil exports to its neighbor should North Korea conduct a sixth nuclear test.
In a recent interview with Bloomberg that received some criticism, Trump said he'd be "honored" to meet with Kim Jong-un "under the right circumstances".