Former U.S. Vice President Walter Mondale, who as ambassador to Japan in the 1990s worked on a plan to close U.S. Marine Corps Air Station Futenma in Okinawa, backs the plan to shift the base further north to the coastal area known as Henoko.

In an interview ahead of the 20th anniversary of a bilateral agreement to return the land used for Futenma, Mondale, 88, also emphasized that there is a need to keep U.S. Marine units stationed in the prefecture, given Japan's tensions with China in the East China Sea.

"It's a shock to me to see, here we are 20 years later, talking about this stuff," said Mondale, who was U.S. ambassador from 1993 to 1996 after serving as vice president under President Jimmy Carter.