TOKYO (Reuters) - Pope Francis appealed on Monday to world leaders to ensure that nuclear weapons are never used again, a day after he visited Hiroshima and Nagasaki, the only cities ever to be hit by atomic bombs.

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Nuclear disarmament has been a key theme of the pope’s trip to Japan, a country not only haunted by the memory of the two attacks that ended World War Two but also alarmed by the nuclear programme and missile tests of nearby North Korea.

“(I) invite all persons of good will to encourage and promote every necessary means of dissuasion so that the destruction generated by atomic bombs in Hiroshima and Nagasaki will never take place again in human history,” Francis told dignitaries including Japan’s Prime Minister Shinzo Abe.

Abe said Japan was committed to a world free of nuclear weapons.

In an apparent reference to the tensions with North Korea, Francis said dialogue was “the only weapon worthy of man and capable of ensuring lasting peace”.

Pyongyang has conducted seven missile tests since U.S. President Donald Trump met North Korean leader Kim Jong Un earlier this year. One of North Korea’s top nuclear negotiators said on Friday it would be the fault of the United States if diplomacy on the issue broke down.

Francis backs a U.N. treaty aiming to ban nuclear weapons and says even their possession for the purpose of deterrence is immoral.

Nuclear devastation was also a topic of the pope’s meeting on Monday with Emperor Naruhito.

A palace spokesman said Francis had told Naruhito that he recalled as a nine-year-old boy in Argentina how his parents had wept on hearing of the attacks on Hiroshima and Nagasaki and how this had left a lasting impression on him.

NUCLEAR ENERGY

Francis, who turns 83 next month, also expressed concern on Monday about future energy sources as he comforted victims of the 2011 Fukushima nuclear reactor disaster, noting a call by Japan’s Catholic bishops to abolish nuclear power outright.

Around 18,000 people died or were classified as missing after a massive earthquake set off a tsunami - in some places 30 meters high - destroying a wide swath of Japan’s northeastern coast and triggering a nuclear meltdown at the plant.

Francis has also been outspoken in his opposition to the death penalty, still used in Japan.

A Mass he said at a Tokyo stadium on Monday was attended by Iwao Hakamada, an 83-year-old man who spent 46 years on death row before being released and was baptized during his time in prison, local church officials said.

Francis ends his four-day trip to Japan on Tuesday.