Speaking at the Vatican on Monday, Pope Francis also urged the world to support dialogue between Pyongyang and South Korea.
In his annual "state of the world" speech, he said the nuclear war could be triggered by "some chance and unforeseen circumstance".
He added: "It is of paramount importance to support every effort at dialogue on the Korean peninsula in order to find new ways of overcoming the current disputes, increasing mutual trust and ensuring a peaceful future for the Korean people and the entire world."
Over the past year, Pyongyang has made big advances in being able to strike the US with a nuclear weapon.
North Korean Leader Kim Jong Un just stated that the "Nuclear Button is on his desk at all times." Will someone from his depleted and food starved regime please inform him that I too have a Nuclear Button, but it is a much bigger & more powerful one than his, and my Button works!
— Donald J. Trump (@realDonaldTrump) January 3, 2018
Kim Jong-un, the North Korean leader, claimed to have a nuclear button at the ready and Mr Trump dismissed the taunt by tweeting that the US button at his disposal was bigger and more powerful.
Meanwhile, the Catholic leader also criticised President Donald Trump for recognising Jerusalem as Israel's capital, calling for the status quo to be respected.
The pope addressed envoys from more than 180 countries a day before North Korea and South Korea held talks to address North Korea's participation in the Pyeongchang 2018 Winter Olympics.
The countries met on Tuesday for their first talks in more than two years.
