Pope Francis has urged for the abolition of nuclear weapons, saying their mere possession is perverse and indefensible.

Key points: The Pope says nuclear weapons are "not the answer" to peace

The Pope says nuclear weapons are "not the answer" to peace The 82yo's stance is a break from previous pontiffs who said they were a "necessary evil"

The 82yo's stance is a break from previous pontiffs who said they were a "necessary evil" Atomic bombs killed 74,000 people in Nagasaki and 140,000 people in Hiroshima

On his maiden visit to Japan, the Pope spoke on Sunday (local time) at Nagasaki's Atomic Bomb Hypocenter Park, ground zero of the bomb the United States dropped on August 9, 1945, instantly killing 27,000 people.

In a highly symbolic visit to the Japanese city devastated by the nuclear attack, the Pope said nuclear weapons were "not the answer" to a desire for security, peace and stability.

"Indeed they seem always to thwart it," he said.

At least 74,000 people died from the atomic bomb unleashed on the city in western Japan in World War II — just three days after the world's first nuclear attack hit Hiroshima and killed at least 140,000.

"This place makes us deeply aware of the pain and horror that we human beings are capable of inflicting upon one another," he said.

Hundreds of people in white waterproofs sat in torrential rain to hear the Pope's speech, next to the emblematic photo of a young boy carrying his dead baby brother on his back in the aftermath of the attack.

Crowds waited in the rain to hear the Pope speak. ( AP: Gregorio Borgia )

He laid a wreath of white flowers and prayed silently, unprotected from the lashing downpour.

Two survivors of Nagasaki, 89-year-old Shigemi Fukahori and 85-year-old Sakue Shimohira, handed the wreath to the pope.

Ms Shimohira, who was 10 at the time of the attack, talked about the terror of the bomb.

"My mother and older sister were killed, charred. Even if you survived, you couldn't live like a human or die like a human … It's the horror of nuclear weapons," she said

Arms trade 'affront crying out to Heaven'

Pope Francis took aim at what he called the "perverse dichotomy" of nuclear deterrence, saying that peace was incompatible with the "fear of mutual destruction or the threat of total annihilation".

This marked a break with past pontiffs — in a 1982 UN speech, Pope John Paul II had described nuclear deterrence as a necessary evil.

"Here, in this city, which witnessed the catastrophic humanitarian and environmental consequences of a nuclear attack, our attempts to speak out against the arms race will never be enough," the 82-year-old Pope said on Sunday.

Resources spent on the "arms race", he said, should be used for development and protection of the environment, instead.

"In a world where millions of children and families live in inhumane conditions, the money squandered and the fortunes made through the manufacture, upgrading, maintenance and sale of ever more destructive weapons, are an affront crying out to heaven," he said.

The Pope also restated his support for a 2017 treaty to ban nuclear weapons agreed by nearly two-thirds of UN members, but opposed by big nuclear powers who say it could undermine nuclear deterrence, which they credit with averting conventional war.

"The possession of nuclear and other weapons of mass destruction is not the answer (to longings for peace)," he said.

"Our world is marked by a perverse dichotomy that tries to defend and ensure stability and peace through a false sense of security sustained by a mentality of fear and mistrust.

"Peace and international stability are incompatible with attempts to build upon the fear of mutual destruction or the threat of total annihilation."

Last August, the United States pulled out of one landmark strategic arms accord, the Intermediate-Range Nuclear Forces Treaty (INF), citing violations by Russia that Moscow denies.

Nuclear experts said it also appeared doubtful that agreement on a full-fledged replacement for the New START nuclear arms control treaty between Russia and the United States would be in place before it expired in February 2021.

The mushroom cloud over Nagasaki, taken 15 minutes after the plutonium bomb exploded on August 9, 1945 ( Hiromichi Matsuda/Nagasaki Atomic Bomb Museum )

Pope Francis is due to visit Hiroshima and meet survivors of the atomic attack, known in Japanese as hibakusha, at the world-famous Peace Memorial in the city synonymous with the horror of nuclear war.

About 78,000 people were killed instantly at Hiroshima, with an estimated total 400,000 more eventually dying of radiation illness and injuries caused the US-dropped bombs at both Hiroshima and Nagasaki.

AFP/Reuters