The shift of 8,600 U.S. Marines from Japan to Guam, a hot political issue in Tokyo, is on track for 2014, a U.S. official said.

David Bice, executive director of the Joint Guam Program Office administering the build-up, was speaking to Reuters after the release of a study on how the move will affect the North Pacific island's ecological balance, including large coral reefs.

The shifting of the U.S. forces, which at its peak could add 41,000 to the U.S. territory's population of about 175,000, would bring more that $10 billion in construction and investment.

A final decision on the relocation is due in September.

"We have not had any discussion about delaying the U.S. forces. The target for the completion remains 2014," Bice said.

"We are going to stay with the capacities of Guam's infrastructure. If we find ways to speed up the construction or the sequencing, we will do so."

Ecological concerns have affected the project's progress.

The U.S. military has put off a decision on the site because of a wharf for visiting nuclear-powered aircraft carriers that could damage coral.

There are also local concerns that a firing range would limit access to a historic site at Pagat in the northeast.

U.S. President Barack Obama signed a law last month to allocate $50 million to upgrade the Port of Guam, with a further $50 million from a USDA loan to help fund construction.

Broader agreement

The move is part of a broader 2006 accord to reorganise U.S. troops in Japan, including relocation of the Marines' Futenma airbase on Okinawa to a quieter part of that Japanese island.

Okinawans associate Futenma with noise, pollution and crime and Japan's previous prime minister, Yukio Hatoyama, raised then dashed their hopes that the base would be moved entirely off the island. He quit after being seen to have mishandled the issue.

Japanese media last month said Washington had abandoned the 2014 target to move to Guam and reported that any delay would also push back the base's relocation on Okinawa.

There have been concerns that Guam lacks the ports, roads and facilities to support the move by the target date.

The Final Environmental Impact Statement, released late last month by the U.S. Navy, estimated about 9,000 dependants would accompany the Marines. A further 600 military personnel and 900 dependants would come as part of an Army Missile Defense Force.

Held by the Japanese for three years during World War Two, Guam boasts large reefs, some of which would be destroyed by construction at any of the proposed sites for the carrier wharf.

The study said officials were considering options to limit damage, including construction of an artificial reef.

Reuters