NAHA (Kyodo) -- Sand deposition for work reinforcing soft soil as part of the U.S. military base transfer within Okinawa Prefecture began Saturday, Japan's Defense Ministry said, marking progress in the controversial plan.

As part of land reclamation at the relocation site for U.S. Marine Corps Air Station Futenma, sand is being embedded in the soft ground on the Oura Bay side of Henoko before the installation of approximately 70,000 piles into the seabed at depths of up to 70 meters.

At about 1:45 p.m., a year to the day since the government took the unprecedented step of overriding the Okinawa prefectural government to greenlight its modified landfill plan, a digger on a boat took sand and soil from a neighboring boat and dumped it in the bay.

In a post on X, formerly Twitter, the ministry hailed the development as a "major step forward for the full return of Futenma air station," adding it will continue to do its utmost to achieve the base transfer.

While the ministry asserts that the site will be stabilized to support the base, the project may encounter challenges due to some areas reportedly reaching depths of up to 90 meters below sea level.

The government submitted a revised landfill plan to the Okinawa governor in April 2020 to address the issue of soft soil. After the Okinawa prefectural government refused to approve it, the central government granted approval in its place on Dec. 28, 2023.

Construction work is expected to take until around April 2033, while arrangements to hand the facility over to the United States are projected to take an additional three years.

Tokyo and Washington agreed in 1996 to return the land used for Futenma, located in a densely populated area of Ginowan on Okinawa's main island in southern Japan.

The Japanese government selected the coastal Henoko area in Nago as the relocation site in 1999, but the plan has been delayed by political wrangling and strong local opposition.

The central government has maintained that the relocation plan is the "only" solution to removing the dangers posed to the local community by the Futenma base without undermining the perceived deterrence provided by the Japan-U.S. alliance.