Amid local protests, construction of helipads for U.S. military forces resumed in Okinawa on Friday in preparation for the return to Japan of part of the U.S. military's largest base in the prefecture.

Riot police have been deployed around the Northern Training Area as local residents continue protests against the construction.

The Defense Ministry's Okinawa Defense Bureau started moving construction material and equipment into the area last week, prompting Okinawa Gov. Takeshi Onaga to warn of damaged relations between the central government and the people in Okinawa, which hosts the bulk of U.S. military facilities in Japan.

On Thursday, the Okinawa Prefectural Assembly demanded the central government halt the project, citing the local population's concerns regarding safety and noise if Osprey aircraft use the helipads to be built in the training area. The Osprey is a tilt-motor aircraft that can land and take off like a helicopter and fly like an airplane.

The United States agreed in 1996 to return to Japan about 4,000 hectares, or almost half of the training area straddling the villages of Kunigami and Higashi in northern Okinawa Island, in exchange for relocating helipads from the portion of the base to be returned to the area retained.

Two of the six helipads have already been completed. But the work, which began in 2007, has been suspended due to the protests.