The United States Defense Department says the relocation of around 8,600 marines and 9,000 of their dependents to Guam from Japan will take until 2016, two years longer than expected due to a lack of facilities on the island.
The move is part of a broader 2006 accord to reorganise U.S. troops in Japan, including the relocation of the marines' Futenma airbase on Okinawa.
The department says the military populations will not be relocated to Guam until the needed facilities are constructed.
There had been concerns that Guam lacked the ports, roads and facilities to support the move by the target of the end of 2014.
An official says delays to funding or construction pacing could further push out the transfer date.
The Department also says a planned Air and Missile Defense Task Force, which would have involved moving a further 600 military personnel and 900 dependents, will not go ahead.