Japan's government has begun preparatory work to build another seawall at the planned relocation site for the US Marine Corps Futenma Air Station in the southern prefecture of Okinawa.

Defense Ministry sources told NHK that a test to drive piles began on Monday at Oura Bay in Nago City. Piles are needed to reinforce the seabed, which has been found to be soft. Concrete seawalls will be built around the planned reclamation area.

The work is part of the government's plan to transfer the US base from Okinawa's densely populated city Ginowan to Henoko in Nago City.

The government and the prefecture were previously due to hold talks before reclamation works start at Oura Bay.

But the government informed the prefecture last month that talks were canceled and construction of a new seawall would start on August 1 or later. It said that there had been enough discussions.

Video footage taken from an NHK helicopter shows piles unloaded from a crane ship into the sea and workers around the piles.

The Okinawa prefectural government, which opposes the relocation, said it is regrettable that the central government unilaterally decided the date to start the construction work. It also said the move to drive piles into the sea is not a test but the start of reinforcement work.

The prefecture is asking the government to cancel the test and the construction work.