The government on Tuesday ratcheted up its efforts to facilitate construction of more seawalls at the planned relocation site of a U.S. air base in Okinawa, a day after the local governor expressed his frustration at the heavy U.S. presence in the prefecture in a meeting with the U.S. ambassador to Japan.

The government started delivering crushed rock to the site by sea, a move aimed at enabling workers to bring in construction materials to the planned landfill area more efficiently than using trucks. But the work saw protesters on boats seek to hinder the operation.

Okinawa Gov. Takeshi Onaga told U.S. Ambassador to Japan William Hagerty on Monday that the heavy local U.S. military presence and the plan to move a key U.S. air base within Okinawa amount to "discrimination" against the island prefecture.

Despite only accounting for 0.6 percent of the country's land mass, Okinawa hosts about 70 percent of the total area of land exclusively used by U.S. military facilities in Japan.

Hagerty told the governor he is committed to working with the Japanese central and local governments on reducing Okinawa's base burden.

Defense Minister Itsunori Onodera reiterated Tuesday that transporting the materials by sea would reduce air pollution and noise.

Construction work is now under way for the relocation of U.S. Marine Corps Air Station Futenma in Ginowan to the Henoko coastal area of Nago, in accordance with an agreement reached between Washington and Tokyo in 1996.

Tuesday morning, ships carrying crushed rocks equal to around 50 10-ton truckloads docked alongside existing seawalls on the northern side of the area adjacent to the Marines' Camp Schwab.

The materials will be used to build seawalls on the southwestern side of the area, work on which started earlier this month.

Under the plan to transfer the functions of the Futenma base from the crowded residential area of Ginowan to the planned site, the central government is scheduled to reclaim around 157 hectares of land in waters off Henoko and construct a V-shaped runway.

The plan comes despite Gov. Takeshi Onaga's administrative guidance asking the Defense Ministry to suspend transport of the materials by sea until a consensus had been reached.

The central and Okinawa governments are fighting a court battle after Onaga, a vocal opponent to the transfer plan, filed a fresh lawsuit in July, seeking a halt in construction at the Futenma base relocation site.

Tokyo and Washington agreed on the return of the land for the contentious base in 1996 and announced in 2006 a road map for realigning the U.S. military in Japan, which included relocating the airfield to Henoko.

Meanwhile, the ministry's Okinawa Defense Bureau said Tuesday 10 colonies of coral designated as species feared to be extinct have been found at two points in the waters off Henoko.