The Japanese government has “hidden from the public” that a controversial U.S. military base relocation within Okinawa is planned in an area larger than disclosed, a newly obtained U.S. briefing document shows.

The central government aims to relocate U.S. Marine Corps Air Station Futenma from a residential area in Ginowan to the less populated Henoko district in Nago, both on Okinawa Island. It was recently revealed that the replacement facilities will be built not only in the coastal area as publicized, but inland as well.

This is corroborated by a document compiled by the U.S. military in February 2010 as a briefing paper for key Congress members and listing the “political challenges” involved.

The paper, disclosed Saturday, cites the planned construction of facilities in the inland area of Henoko as among the “sensitivities” for the Japanese government.

It also says development of the inland area, referred to as the “area west of Route 329,” is being kept “hidden from (the Japanese) public.”

The government’s secrecy is certain to backfire by exacerbating Okinawans’ bitterness on the relocation issue, and the heavy presence of the U.S. military in the prefecture.

A separate 2008 internal U.S. government document, a copy of which was also obtained by Kyodo News, shows more than 30 facilities, including housing and an athletic field for U.S. servicemen, will be constructed in a wooded area west of Route 329.

The wooded area is currently part of U.S. Marine Corps Camp Schwab. The central government has only informed the public that the area, currently employed for U.S. military drills, will be used to supply soil for landfill work related to construction of the Futenma replacement facility in the coastal area.

According to a U.S. government source, the 2010 briefing document reflected that the Defense Ministry was negative about releasing the information. The source also said U.S. government officials have been “baffled” that the Defense Ministry is still denying that plans exist to build facilities in the inland area.

Observers say this is not the first time the Japanese government has tried to withhold key information regarding U.S. bases in Okinawa.

Leading up to the deployment of MV-22 Osprey aircraft at Futenma in 2012, the government denied the U.S. Marine Corps’ deployment plan for more than 10 years, fearful of fueling anger among local residents, despite Washington requesting that Tokyo make the details public.

The Osprey issue is also outlined in the 2010 briefing paper. Although the Japanese government knew about the plan, it repeatedly told the public it was not aware of the details until the U.S. Defense Department announced in June 2011 that the tilt-rotor aircraft, which have a checkered safety history abroad, would be deployed at the Futenma base in the latter half of 2012.

The briefing paper noted the Japanese government was acting like it “cannot mention” the Osprey issue.

A U.S. government source, referring to the Henoko development plan, said, “The Japanese government is repeating the same thing it did at the time of the Osprey deployment,” adding the United States is growing frustrated with Japan’s handling of the issue.