Japan and the United States have began making adjustments to increase to $3.5 billion from $2.8 billion the costs Tokyo will shoulder to transfer U.S. Marines from Okinawa to Guam, sources said.

The two governments agreed in 2009 on Japan's contribution of $2.8 billion to cover the cost of constructing buildings for a command center and other facilities, including schools, on the Pacific island.

But in bilateral talks that began in February, the United States asked Japan to increase its cost-sharing under growing pressure from Congress to cut expenditures.