Last week, it said the United States and South Korea should "execute" those involved in the purported plot.
Japanese officials have described the first telephone call between new South Korean President Moon Jae-in and Japanese Prime Minster Shinzo Abe as "positive and meaningful", with the two agreeing to cooperate closely to deal with North Korea.
Pyongyang accused South Korea and the United States of plotting to assassinate the leader using biochemical poison.
The CIA created its first single country-focused mission center, pulling together resources from a range of units to collect and analyze information on Pyongyang's nuclear weapons and long-range ballistic missile technology that could extend its military threat across the Pacific.
The official, who was next to Moon as he spoke with Trump, also said the USA president mentioned the trade issue "in a light fashion, mentioning it on a principle level" and added that the "focus was on the North Korean nuclear issue".
The U.N. Mission said the Ministry of State Security has declared that a "Korean-style anti-terrorist offensive will be commenced to mop up the intelligence and plot-breeding organizations of the USA and South Korea".More news: Other world leaders in addition to Trump are anxious about North Korea
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The KCNA report did not say when the alleged plot was stopped.
Moon's office said Thursday that the comment came in a phone call between the two leaders that also touched on a controversial 2015 agreement over compensating South Korean women forced into sexual slavery by Japan's military in World War II. But it's worth noting that North Korea has a long history of its own ripped-from-a-bad-movie assassination plots.
As KCNA announced the allegations last week, it also said North Korea was embarking on a campaign to root out suspected foreign agents.
There was, for example, the 1968 attempt to kill South Korea's president.
"North Korea is a very hard country to analyze, since most information must be deduced from satellite images, what little they publish officially and from defectors", Cook wrote in the Diplomat on May 1.
Now, Han said, the North wants anyone involved to be handed over for punishment.