Pope Francis (right) meets with Japan's Emperor Naruhito at the Imperial Palace in Tokyo on Monday. Photo: AFP

Pope Francis on Monday urged renewed help for victims of Japan's 2011 earthquake, tsunami and Fukushima disaster, warning "nobody can start over alone" as he noted "concern" over nuclear power.

On the penultimate day of his Japan trip, the pontiff heard the stories of those who survived the so-called "triple disaster" - a 9.1-magnitude earthquake that triggered a devastating tsunami and sent a nuclear plant into meltdown.

In an emotional meeting, he embraced 17-year-old Matsuki Kamoshita, who spoke powerfully of his experience as an evacuee, saying he was so badly bullied he "wanted to die."

The 82-year-old Argentine paid tribute to those who rushed to help following the disaster, but warned that more was needed.

"No one 'rebuilds' by himself or herself; nobody can start over alone. We have to find a friendly and fraternal hand, capable of helping to raise not just a city, but also our horizon and our hope."

Some 18,500 were killed or are missing after the disaster. The waves swept away homes and farms, and engulfed the cooling systems at the Fukushima Daiichi power plant, triggering the worst nuclear disaster since Chernobyl.

Nearly half a million people fled their homes in the first days after the quake and even today, roughly 50,000 remain in temporary housing.

Survivors shared painful memories of the disaster, with Toshiko Kato recalling finding her home had been swept away by the waves.

The head of the world's 1.3 billion Catholics stopped short of intervening in the debate over nuclear power in Japan, but said there were "important decisions" to be made about future energy sources.

"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.

After the meeting, Kamoshita told reporters that he had been "deeply touched" to discover the pope remembered meeting him at the Vatican earlier this year.

Francis later met with youth at a Tokyo cathedral, where he repeated a warning he sounded on the pitfalls of technology.

He called on young people to reach out to others and not isolate themselves.