Lobbyists in Okinawa Prefecture are reporting damage to coral from concrete blocks placed on the seabed for a drilling survey ahead of the planned construction of a U.S. military base there.

The Defense Ministry has dropped concrete blocks weighing between 10 and 45 tons at dozens of locations off the coast of Henoko in Nago to create anchors for floats demarcating restrictions in the survey area, according to a report the ministry's Okinawa Defense Bureau relayed to the prefectural government.

Tokyo plans to reclaim land at the site and to build a new base there for troops and aircraft relocated from Futenma air station, despite objections from many residents.

In a diving survey earlier this month, the lobbyists — a group named Herikichi Hantai Kyogikai ("Council opposed to the offshore heliport base") — reported coral damage at five locations.

They produced a photograph purporting to show one of the concrete blocks grazing the coral.

The prefectural government said it is checking whether its approval is required for placing concrete blocks on the seabed under its rules on fisheries.

But an official at the ministry's bureau said it believes prefectural approval is not required. The official said the blocks will be lifted from the seabed once the drilling survey is complete.