TOKYO (Reuters) - The United States and Japan held a ceremony on Wednesday marking the U.S. military’s return of nearly 10,000 acres (4,046 hectares) of land on Okinawa island to the Japanese government, the largest transfer since 1972.

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Resentment on the southern island has simmered for years among residents opposed to the U.S. Marines’ Futenma air base there. They want the base moved off the island but the central government aims to relocate it to a less-populated part of Okinawa.

Prime Minister Shinzo Abe and the U.S. ambassador to Japan, Caroline Kennedy, attended the ceremony in Tokyo, with another event on Okinawa planned for Thursday.

The U.S. military said in a statement Japan had made sufficient progress in building helipads and access roads to consolidate military training in other areas and allow for the return of the 9,909 acres.

The returned land is part of the Northern Training Area, or Camp Gonsalves or the Jungle Warfare Training Center, and is the largest U.S. installation in Japan with 19,300 acres of land.

Okinawa, which was under U.S. occupation until 1972, hosts the bulk of the approximately 50,000 U.S. military personnel in Japan.

Although agreed in 1996, the return of the land was delayed by protesters blocking the construction of helipads. The Japanese government recently resumed work at the site.

Japan’s top court on Tuesday ruled in favor of the government plan to relocate the Futenma base to another part of the island, dealing a blow to islanders’ efforts to get rid of it altogether.

Tension between Okinawa authorities and the U.S. military increased this month when a MV-22 Osprey aircraft crashed southwest of Okinawa, the first accident involving the aircraft in Japan.

Okinawa residents worry about crashes in populated areas.

Japanese media reported Okinawa governor Takeshi Onaga planned to skip the ceremony on Thursday and instead attend a protest against the U.S. military presence on the island.