A Japanese human rights group Friday called for the immediate release of an activist detained for nearly five months in connection with his opposition to U.S. bases in Okinawa Prefecture.

Akira Maeda of the Japanese Workers' Committee for Human Rights, slammed the government for continuing to detain Hiroji Yamashiro, head of the Okinawa Peace Action Center, during a session of the U.N. Human Rights Council in Geneva.

"Mr. Hiroji Yamashiro . . . has been detained for 140 days on relatively minor charges, triggering accusations that the Japanese government is trying to silence him," he said. "Yamashiro's continued incarceration contravenes the International Covenant on Civil and Political Rights."

Yamashiro, 64, has led groups objecting to the relocation of the U.S. Marine Corps' Air Station Futenma within the prefecture.

He was arrested in October on suspicion of cutting barbed wire near a U.S. military helipad construction site in Higashi, in northern Okinawa. He has also been charged with injuring a Defense Ministry official and obstructing relocation work at another U.S. Marine base in Nago.

Amnesty International Japan has also called for Yamashiro's immediate release, saying he does not meet the criteria for being detained as the chances of him destroying evidence concerning his alleged crimes are very low.