Protesters in Okinawa blocked a van attempting to deliver a report to a local prefecture on the relocation of the U.S. Marine base Futenma.

The Japanese Defense Ministry said it will send by mail to the prefecture in the Okinawan capital of Naha the government's Environmental Impact Assessment regarding the move.

The proposal is to relocate the helicopter base from the densely populated civilian area in Ginowan, north of Naha, to an environmentally sensitive but remote area near another U.S. military base.

Around 200 protesters in Naha stopped the van driver from leaving his vehicle, a report by Kyodo news agency said.

The blockade is the latest attempt by protesters and others opposed to having U.S. Marine bases on Okinawa, one of the largest islands in the Ryukyu Island chain that curls south of Japan for more than 600 miles toward Taiwan. The islands are strategically important to the United States because of their proximity to China.

Okinawa is less than 900 square miles and a population of around 1.5 million people. But the more than a dozen American bases have one-third of the 40,000 U.S. servicemen stationed in Japan.

The U.S. bases are a legacy of World War II when U.S. forces captured the island chain on their way to defeating Japan.

In 1972 the U.S. government returned the islands to Japanese administration but the

Treaty of Mutual Cooperation and Security with Japan allows the United States to maintain a large military presence.

However, Okinawa's population has put increasing pressure on successive Japanese governments to do a deal with the United States.

Protesters want to move many of the bases, if not off the island at least to less-populated areas to minimize confrontations -- including assaults and traffic accidents -- between civilians and off-duty military staff. The rape of a 12-year-old girl by U.S. servicemen in 1995 set of large protests in Okinawa.

But there also is a major concern over the safety of helicopters out of necessity over civilian area. In August 2004, a U.S. helicopter crashed into the grounds of nearby Okinawa International University.

There were no casualties or injuries, except minor injuries to the three U.S. crewmembers. However, the fiery crash reinforced the "move Futenma" protests and up to 30,000 people reportedly attended rallies the next month in the streets of Ginowan.

Despite an agreement in October 2005 between the U.S. and Japanese governments to eventually move Futenma, progress has been slow.

Even though many people in Ginowan want the base relocated, they also understand thousands of jobs would be lost to the area.

The local population surrounding the remote U.S. base Camp Schwab, where Futenma could be relocated, is against the move to an area near the fishing village of Henoko. They say they are concerned about the environment which would, they claim, be damaged by building a heliport in the surrounding countryside.

The report being mailed to the prefecture in Naha is the government's reaction to those environment concerns.

In October, U.S. Defense Secretary Leon Panetta said he was pleased with Japan's schedule to deliver the final environmental report by the end of the year, a report in the Stars and Stripes said. Panetta said he hoped that the report would clear the way for a quick move of the base away from Ginowan.

"It's very important to us that the environmental impact study be completed before the end of the year," Panetta said during a news conference in Okinawa. "And as to the other steps involved in this process, the best I could tell you is 'as soon as possible."