The Japanese government on Tuesday began constructing seawalls for a replacement facility for the U.S. Air Station Futenma in Okinawa Prefecture, taking a further step in pushing ahead with the controversial relocation plan.

Despite persistent local opposition to the relocation of the base from a residential area in Ginowan to a coastal area in Nago, the state is going ahead with dumping building materials into the sea and plans to launch full-fledged landfill work inside the seawalls possibly in the first half of next year. The prefecture wants the base moved out of Okinawa altogether.

"I'm convinced that the start of the construction marks a steady first step toward realizing the complete return of the Futenma airfield," Defense Minister Tomomi Inada said in a statement, referring to a U.S.-Japan agreement on the return of the land occupied by the base to Japan.

Okinawa Gov. Takeshi Onaga is considering taking legal action, including filing a lawsuit, to block the construction work. He has claimed the central government needs to obtain his permission for the work, and the last such permit given to the state already expired at the end of March.

He has also threatened to retract the approval for the landfill work given by his predecessor. While a court has already ruled his annulment of the approval was illegal, he can still seek retraction.

Japan and the United States agreed on the return of the land occupied by the U.S. airfield in 1996. The construction of the replacement facility began in October 2015.