The United States is footing more of the bill for overseas bases in Germany, Japan and South Korea even as the military reduces the number of American troops in Europe and strategically repositions forces in Asia, a congressional report says.

The exhaustive, yearlong investigation by the Senate Armed Services Committee focused on costs and burden-sharing as the United States spends more than $10 billion a year to back up the U.S. military presence overseas, with 70 percent of the amount expended in the three nations. The figure does not include military personnel costs.

The panel’s report released Wednesday found the financial contributions by those host countries lagging behind costs or increases in U.S. spending. The report identified inherent problems and missteps in the compensation system as the U.S. returns a growing number of its upgraded facilities on foreign land to the host countries.

The report insisted that the American presence in the countries is vital, especially with bellicose threats from a nuclear North Korea, China’s military growth and Germany’s partnership with the U.S. in NATO. However, the Pentagon is now facing the pressure of deficit-driven, smaller budgets while looking to scale back or close overseas and U.S. domestic bases.

The cost arrangements with allies have drawn greater congressional scrutiny — and frustration.

“The growth in our share has really been pretty stunning. And I think we’ve got to stop that direction,” Sen. Carl Levin, D-Mich., chairman of the committee, said in an interview. “We cannot carry these greater and greater and greater percentages of the costs of maintaining these facilities. The ones that we give back, we’re going to have to be given appropriate consideration for the improvements, and the ones that we keep will have to have a much fairer burden-sharing than has been the case in the last 10 years.”

The size of the U.S. force in Germany is expected to drop in the coming year as the Army plans to deactivate one brigade combat team next year and possibly reduce the force even further with a cut of around 2,500 soldiers. Currently, the U.S. maintains 48,000 active-duty personnel in Germany at a cost of $4 billion.

The amount does not include military personnel costs of $3.9 billion.

The troop reductions have resulted in excess military facilities. However, instead of getting cash for the buildings, the U.S. has been negotiating residual value, in-kind payments of services or facilities with a cash value.

Cash payments would be placed in an account and only used for maintenance, repair and environmental restoration. In-kind payments can be directed to military construction.

The committee identified some questionable military construction projects from this arrangement. About $200,000 of residual value, in-kind payments was directed to add sunrooms to senior officer homes in Stuttgart, Germany, in 2007.

“Before the 200-square-foot additions, the homes were each 2,490 square feet, well above DoD’s minimum size standard,” the report said.

The report found that the U.S. has negotiated the return of more than 100 facilities to Germany since 2005, yet they haven’t been included in the residual value settlements between the two countries. Their current value is nearly $1.1 billion.

“It’s really been sloppy and it’s going to have to change even without the budget crunch,” Levin said.