TOKYO -- Pope Francis voiced concern about nuclear power Monday after meeting with victims of Japan's 2011 nuclear disaster, saying the development of future energy sources must take environmental considerations into account.

Francis didn't explicitly back a ban on nuclear energy during his emotional encounter with victims. But he recalled that Japan's Catholic bishops called for the abolition of nuclear power plants in the aftermath of the "triple disaster," in which three reactors at a nuclear plant in Fukushima melted down after an earthquake triggered a tsunami.

The meltdown coated the area in radioactive fallout and at one point forced the displacement of 160,000 people. Nine years later, more than 40,000 people still can't return home.

After comforting some of the evacuees who gathered in Tokyo, Francis said the Fukushima accident will not be fully resolved until the scientific, medical and societal concerns it raised are addressed.

"In turn, this involves, as my brother bishops in Japan have emphasized, concern about the continuing use of nuclear power; for this reason, they have called for the abolition of nuclear power plants," he said.

Going forward, he said, "important decisions will have to be made about the use of natural resources, and future energy sources in particular."

During his first full day in Japan on Sunday, Francis visited Nagasaki and Hiroshima -- where two U.S. atomic bombs were dropped in World War II -- and said both the use and possession of nuclear weapons was "immoral" and that the Cold War-era doctrine of deterrence was a dangerous waste of resources.

He has not articulated a formal position on nuclear power, but the Vatican has previously called for the "safe, secure, and peaceful, development and operation of nuclear technologies." Francis, however, has made environmental concerns a pillar of his papacy and has now heard first-hand from Hiroshima and Fukushima survivors of the health and environmental effects of both intended and accidental exposure to nuclear fallout.

The government of Prime Minister Shinzo Abe responded Monday to Francis' Hiroshima message, saying Japan seeks a nuclear-free world 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.

Abe has sought to restart as many nuclear power reactors as possible to keep the industry alive, especially as the government seeks to showcase its recovery ahead of the Tokyo Olympics next year.

Areas that used to be under the no-go zone in Fukushima have opened following decontamination efforts, prompting people to return home and resulting in cuts to government financial support for evacuees.

One of those evacuees, Matsuki Kamoshita, addressed Francis at the encounter Monday to appeal for an end to nuclear power.

Kamoshita, a 17-year-old high school student from Iwaki on the eastern coast of Fukushima, wrote to the pope last year begging that he visit Fukushima to see for himself the impact. He was rewarded with a papal audience at the Vatican, and on Monday a chance to address the pope in public to tell his story.

In his speech, Kamoshita lamented that the government had "given up" on housing evacuees while continuing to pursue nuclear power as a state policy when the safety concerns are not resolved.

"It will take many times longer than my lifetime to restore the contaminated land and forests," he told the pope. "So, for us who live there, adults have a responsibility to explain without concealing anything about radioactive contamination, exposure and possible damage in the future. I don't want them to die before us, having lied or not admitting the truth."

Kamoshita asked for the pope to pray that political leaders find another path.

"And please pray with us that people from all over the world will work to eliminate the threat of radiation exposure from our future," he said.

After he finished, he approached the pope, who took him in his arms for a long embrace.

Francis' meeting with the victims kicked off a busy day of activities in Tokyo, including a private audience with Emperor Naruhito, a rally with young people and Mass at the Tokyo Dome.

During the meeting with Naruhito, Francis told the emperor that as a 9-year-old boy in Argentina, he remembered seeing his parents weep at news of the 1945 atomic bombings. He told the emperor that he recalled that memory when he addressed survivors in Nagasaki and Hiroshima, a palace official said.

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.

During his meeting with young people, Francis denounced what he called an "epidemic" of bullying that is afflicting youth in Japan and elsewhere.

"We must all unite against this culture of bullying and learn to say "Enough!" Francis told the students, three of whom recounted the pressures they face in a hyper-competitive society, their feelings of inadequacy and the cruelty they sometimes face from classmates that drives some to suicide.

Francis wraps up his weeklong trip to Asia with a speech Tuesday at Sophia University, Japan's main Catholic university founded by his Jesuit order a century ago.