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TOKYO — Former President Barack Obama said Sunday that negotiations with North Korea on its nuclear weapons program are difficult in part because the country's isolation minimizes possible leverage, such as trade and travel sanctions against Pyongyang.

"North Korea is an example of a country that is so far out of the international norms and so disconnected with the rest of the world," Obama told a packed hall in Tokyo Sunday.

He stressed the effort to get North Korea to give up nuclear weapons remains difficult, but countries working together, including China as well as South Korea and Japan, to combine pressure on North Korea will be better than nations working alone.

Obama noted that past U.S. efforts on Iran's nuclear weapons were more successful because there was more leverage, but there's little commerce and travel with North Korea to being with.

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"That makes them less subject to these kinds of negotiations," he said of North Korea.

Obama was speaking at an event sponsored by a Japanese nonprofit group during his trip to Asia, which included earlier stops in Singapore, New Zealand and Australia. His work after leaving office has been focused on nurturing young leaders.

Obama, welcomed by a standing ovation, said the U.S.-Japan alliance remained strong, and the U.S. committed to defending Japan.

Former President Barack Obama shakes hands with Japanese Prime Minister Shinzo Abe in Tokyo's Ginza shopping district on Sunday. SHIZUO KAMBAYASHI / POOL / EPA

"North Korea is a real threat," he said.

"Our view has always been that we would prefer to resolve these issues peacefully," he said, adding that otherwise "the cost in terms of human life would be significant."

He acknowledged progress on a nuclear-free world will likely take a long time as long as Russia and the U.S. could not agree to start reducing their stockpiles.

Obama also reflected on his 2016 visit to Hiroshima, one of two Japanese cities where the U.S. dropped atomic bombs in the closing days of World War II. His visit was the first by an American president.

Almost all American presidents tend to be relatively popular in Japan, which views the U.S. as its most important ally. But many Japanese particularly appreciate Obama's efforts on denuclearization and remember with fondness his trip to Hiroshima and his message of working toward a world without nuclear weapons.

"It was an extraordinarily powerful moment for me," Obama recalled.