U.S. Forces Japan on Wednesday formally announced that it will return more than half the Northern Training Area Thursday amid growing anger in Okinawa over the recent crash of an Osprey aircraft off the prefecture.

Both Tokyo and Washington trumpeted the return of the biggest land parcel by the U.S. military since Okinawa was reverted to Japan in 1972. A ceremony in the city of Nago in Okinawa was scheduled for Thursday.

The return “represents our continuing commitment” to reducing impacts on citizens in Okinawa, U.S. Ambassador Caroline Kennedy said during a meeting with Prime Minister Shinzo Abe on Wednesday evening.

“This reversion will not only reduce burdens of the U.S. bases but also contribute to invigorating local economies,” Abe said, vowing to further strengthen Japan-U.S. relations.

According to the Defense Ministry, the reversion will reduce Okinawa’s share of U.S. military-exclusive facilities to 70.6 percent from the current 74 percent. Such facilities, which currently occupy 17.9 percent of the main island of Okinawa, will shrink to 14.6 percent, according to the Okinawa Prefectural Government.

The return of about 4,000 hectares of the training area, also known as Camp Gonsalves or the Jungle Warfare Training Center, was established in 1996 under the Special Action Committee on Okinawa agreement, reached following the gang rape of a 12-year-old Okinawan girl by three U.S. military service members. The U.S. agreed to partially return the land to Japan in exchange for allowing the construction of six helipads in the remaining areas.

From the beginning, the reversion process met fierce opposition from local residents and environmental groups, who cite noise and environmental impact on the forested Yanbaru region.

Two helipads were completed in 2014, but construction was suspended after massive protests. The construction resumed in July this year, but sit-in protesters have clashed with riot and prefectural police.

While Tokyo says the reversion will mitigate the burden of Okinawans, critics disagree, saying the deal involves what the U.S. military has called “unusable land” in exchange for new helipads.

Concerns among residents have been heightened because the return comes so soon after the first major accident in Japan involving an MV-22 tilt-rotor Osprey aircraft on Dec. 13. The craft has been called a widow-maker since its use began in Japan due to high profile accidents around the world.

The U.S. military resumed Osprey flights in Japan on Monday — less than a week after the Okinawa accident.

Residents of the Takae district, which is surrounded by the helipads, fear more accidents are possible as training exercises have become more sophisticated, some involving low-altitude flights over residential areas. The U.S. military has been conducting drills with Osprey, using the helipads that were previously completed.

Despite the celebratory tones of the two governments, Okinawa Gov. Takeshi Onaga said he will not attend the reversion ceremony in protest of the early resumption of Osprey flights.

The government says the land return will benefit the villages of Kunigami and Higashi. The two municipalities hope to turn the area into a national park together with the remaining training area, and eventually apply for listing as a UNESCO World Heritage site.

Still, Hiromori Maedomari, professor at Okinawa International University, sees a contradiction in Tokyo’s stance.

“They hope the area will be listed as a World Heritage site, but they are also cutting down trees to offer a training facility for Ospreys. How are they going to explain this contradiction?” Maedomari said.

There is also a potential issue of contamination. Some 85 percent of the land is owned by the central government, but Tokyo will return the rest of the land to the prefecture, the village of Kunigami and residents after a decontamination process that could last a year or more. According to the Status of Forces Agreement, the U.S. military is exempt from decontamination work.

Previously returned U.S. military land was found to be contaminated, and it sometimes took years before decontamination work was completed.

Staff writer Tomohiro Osaki contributed to this report.