Japan's government has begun preparatory work to reinforce the ground at a planned relocation site for a US military base in Okinawa Prefecture.

The government plans to reclaim land off Henoko in Nago City to transfer the functions of the US Marine Corps Futenma Air Station, currently in the densely populated city of Ginowan.

The Okinawa prefectural government opposes the plan, and has refused to approve necessary work to reinforce the soft seabed at Oura Bay.

In December, the central government won a court ruling that allowed the land ministry to override the prefecture's objections.

On Tuesday morning, a work vessel arrived in an off-limits zone sealed off with buoys, and divers were later seen working under water.

Protesters approached the area by boat and raised banners saying Okinawa will decide its future by itself, and that the ocean must be protected.

Defense Ministry sources say a silt screen will be set up in the sea on Tuesday to prepare for building an offshore construction yard to develop a seawall in the area. Full-fledged work to build the yard will begin as early as Friday.

The ministry says it will take about 12 years before all construction work and other procedures can be finished, making it possible to move the Futenma base.

Okinawa Prefecture is urging the central government to abandon the plan and engage in dialogue so that a new solution can be found.