Former President Barack Obama says negotiations with North Korea on its nuclear weapons program are difficult, partly because the country's isolation minimizes possible leverage, such as trade and travel sanctions against Pyongyang.

Obama told a packed hall in Tokyo Sunday that the effort to get North Korea to give up nuclear weapons remains difficult, but countries working together to combine pressure on North Korea is better than nations working alone.

He says past U.S. efforts on Iran's nuclear weapons were more successful because there was more leverage.

Obama is 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.

Obama's work after leaving office has been focused on nurturing young leaders.