Former president Barack Obama has spoken out on the rising tensions with North Korea, describing the country as a “real threat”.
He said the negotiations with North Korea on its nuclear weapons program were difficult, partly because the country's isolation minimises possible leverage, such as trade and travel sanctions against Pyongyang.
Mr Obama stressed the need to work with other countries, including China as well as South Korea and Japan, to combine pressure on North Korea.
He was speaking at a packed hall in Tokyo during his tour of Asia which has included earlier stops in Singapore, New Zealand and Australia.
"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," Mr Obama said.
"North Korea is a real threat. Our view has always been that we would prefer to resolve these issues peacefully," he said.
He also added that otherwise "the cost in terms of human life would be significant."
Mr Obama also noted previous 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 begin with.
"That makes them less subject to these kinds of negotiations," he said of North Korea.
He acknowledged progress on a nuclear-free world will likely take a long time as long as Russia and the US could not agree to start reducing their stockpiles.
Reflecting on his 2016 visit to to Hiroshima, one of two Japanese cities where the US 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 US as its most important ally.
But many Japanese particularly appreciate Mr Obama's efforts on denuclearisation and remember with fondness his trip to Hiroshima and his message of working towards a world without nuclear weapons.
"It was an extraordinarily powerful moment for me," Mr Obama said.
Agencies contributed to this report
