Pope Francis was speaking on board his plane as he flew off to Chile and Peru today.
He was quizzed about the menace of a nuclear conflict after a false missile alert in Hawaii caused panic as tensions continue with North Korea.
The Pope said: “I think we are at the very limit. I am really afraid of this.
“One accident is enough to precipitate things.”
On board the plane, Vatican officials handed out a 1945 photograph showing a Japanese boy carrying his dead brother on his shoulders following the US nuclear attack on Nagasaki.
The Pope told reporters: “I was moved when I saw this. The only thing I could think of adding were the words ‘the fruit of war’.
“I wanted to have it reprinted and distributed because an image like this can be more moving than a thousand words.
“That is why I wanted to share it with you.”
Pope Francis has often flagged the danger of nuclear warfare and in November he appeared to harden the Catholic Church’s stance, saying countries should not stockpile the weapons even as a deterrent.
An emergency alert was sent mistakenly on Saturday to Hawaii’s residents warning of an imminent ballistic missile attack when an employee at the state emergency management agency pushed the wrong button.
State officials confirmed that there was no actual threat to the state.
But for more than a half hour, while the agency struggled to retract the warning, panicked Hawaiians scrambled to find shelter.
The mistaken alert, sent to mobile phones and aired on television and radio, stated: “EMERGENCY ALERT BALLISTIC MISSILE THREAT INBOUND TO HAWAII.
“SEEK IMMEDIATE SHELTER. THIS IS NOT A DRILL.”
The Pope spoke as North Korea agreed to send a 140-strong orchestra to perform at the South’s Winter Olympics next month in a bid to ease tensions over its nuclear programme.
Officials from the two Koreas met at the border village of Panmunjom to discuss Pyongyang sending performers for the first time in 18 years.
The decision to host the orchestra “contributes to improving relations and recovering the cultural homogeneity” between the two Koreas, the South’s ministry said.
North Korea and South Korea separately agreed to hold working talks on Wednesday on the North’s athletes attending the Olympics.
Tyrant Kim Jong-un is continuing his nuclear programme in defiance of UN Security Council and other sanctions .
He has frequently threatened to destroy the US and its two key Asian allies, South Korea and Japan.
But fears of war have eased slightly after the first round of intra-Korean talks in more than two years last week.