Photo/Illutration Pope Francis waves in Popemobile as he arrives for Holy Mass at Tokyo Dome on Nov. 25 in Tokyo. (AP Photo)

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.

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 II but also alarmed by the nuclear program 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 Prime Minister Shinzo Abe.

Abe said Japan is committed to a world free of nuclear weapons but still depends on U.S. nuclear deterrence because of the worsening security environment in the region.

Chief Cabinet Secretary Yoshihide Suga said dependence on the U.S. nuclear umbrella, and even strengthening it, was “realistic and appropriate.”

Abe’s conservative government, which is seeking to amend the postwar pacifist Constitution to allow a full-fledged military, has explained not signing the new U.N. treaty on the prohibition of nuclear weapons by saying it wants to be a “bridge” between nuclear and non-nuclear states.

Abe repeated that Monday in his speech to Francis, saying the government would be “utterly tireless” in seeking dialogue.

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.

Present in the crowd of 50,000 for Francis’ Mass was Iwao Hakamada, a former professional boxer who has become a leading symbol for the anti-death penalty movement in Japan. Hakamada, 83, converted to Catholicism during his decades on death row for murders he says he did not commit.

Francis has said the death penalty is “inadmissible” in all cases, and one of his main messages while in Japan was to “respect all life.” Local organizers confirmed Hakamada was at the Mass, but the Vatican declined to say if the pope met with him as his supporters had hoped.

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