The North Korean leader had attacked Donald Trump days after his speech at the United Nations, in which the USA president said he would "totally destroy" North Korea if the U.S. was forced to defend itself or its allies.
His visit comes in the wake of North Korea's sixth nuclear test on September 3 and recent ballistic missile launches, including two over Japan.
It was the first time for a North Korean leader to issue such a direct statement against a US president, dramatically escalating the war of words between the former wartime foes and raising the worldwide nuclear standoff to a new level.
North Korean leader Kim Jong Un and President Trump.
Donald Trump and Kim Jong-Un are doing their best to make a bad situation in the Korean Peninsula worse.
The United Nations Security Council had also condemned the launch of yet another ballistic missile by the Democratic People's Republic of Korea (DPRK) and called on the country's leadership to halt such activities and explore resumption of honest dialogue on de-nuclearisation.
Though Kim's response referenced Trump's August threats that the United States would unleash "fire and fury" on Pyongyang, that was not the focus of netizens. China is North Korea's main trading partner and conduit for worldwide transactions.
On Thursday, Kim said Trump is "unfit to hold the prerogative of supreme command of a country" who will "pay dearly" for threats he's made.
MCEVERS: Today President Trump struck back with a tweet. Trump has said he won't allow it, although the U.S.so far has not used military force to impede the North's progress.
The statement will further escalate the war of words between the adversaries as the North moves closer to perfecting a nuclear-tipped missile that could strike America. In a one-and-only such test in 1962, the USA launched a nuclear-tipped ballistic missile from a submerged submarine.
Kim said Trump would "pay dearly" for his warning of destruction.
Newly-signed sanctions gave the Trump Administration the power to penalize individual companies and institutions that conduct and finance trade with North Korea.
The explosion of a North Korean hydrogen bomb could be potentially deadly, because it could jam electronic signals and disable technology on planes, ships and submarines in the Pacific.
It's not the first time North Korea has resorted to insults when delivering statements about its enemies.
It imposes a 180-day ban on vessels and aircraft that have visited North Korea from visiting the United States.
Kim said he is "thinking hard" about his response and that he would "tame the mentally deranged U".
It could be that Kim felt that a direct, personal response was crucial because of the harshness of Trump's comments.