Pope Francis said Monday that the world stood on the "edge" of nuclear war and admitted the situation terrified him.

The Argentine was speaking aboard the papal plane on his way to Chile and Peru, the day after a false missile alert sparked panic in the US state of Hawaii, already on edge over fears of a North Korean attack.

Asked about the threat of nuclear war in the wake of a flurry of nuclear and missile tests since last year in North Korea, the pope said: "I think we are at the very edge.

"I am really afraid of this. One accident is enough to precipitate things," he told journalists.

The 81-year old, who has often spoken of the dangers of nuclear weapons, presented the press pack aboard his plane with a harrowing photograph taken in 1945 that shows a young Japanese boy carrying his dead brother.

The child, carried on the boy's back, was killed when the United States dropped a nuclear bomb on Nagasaki.

Francis had written on the back of the image just four words: "The fruit of war".

"I wanted to reprint and distribute it because an image like this can be more moving than a thousand words. That is why I wanted to share it with you," he said.