By Jane Sutton

(Reuters) - Federal and military prisons in the United States could securely hold the remaining 166 Guantanamo prisoners, according to a U.S. report that assessed the logistics but not the politics of closing the controversial detention facility.

The prisoners were captured during counterterrorism operations overseas and have been held at the Guantanamo Bay U.S. Naval Base in Cuba, most without charge for more than a decade.

The report by the Government Accountability Office said 98 federal prisons and six military prisons could safely hold them under the same type of security conditions that exist at Guantanamo.

"This report demonstrates that if the political will exists, we could finally close Guantanamo without imperiling our national security," said Senator Dianne Feinstein, a California Democrat who chairs the Senate Intelligence Committee and released the report on Wednesday.

The 98 federal prisons named in the report already hold 373 people convicted of terrorism-related crimes and "As far as I know there hasn't been a single security problem reported in any of these cases," Feinstein said.

Upon taking office in 2009, President Barack Obama ordered the Guantanamo prison closed within a year, and he said after his re-election in November that he still wants to close it.

Congress blocked the shutdown and put such stringent restrictions on detainee transfers that it has proven nearly impossible to move out the 56 prisoners who have been cleared for release to their home nations or third countries.

Feinstein requested the report in 2008, and there are no plans to move any Guantanamo detainees to the United States, which current law prohibits.

The report does not make any recommendations, and said some of the existing facilities would need to be modified and current inmates may need to be relocated if Guantanamo prisoners were transferred there.

For example, the U.S. Bureau of Prison facilities identified in the report are collectively already 38 percent over capacity. Holding the Guantanamo captives in single cells there could require the bureau to house other prisoners three to a cell, the report said.

The six military prisons listed in the report are only at 48 percent capacity, but Guantanamo captives moved there would have to be segregated since the law prohibits holding members of the U.S. armed forces in "immediate association" with foreign nationals.

Policy changes would also be needed since the federal prisons are not authorized to hold people who are not accused of crimes, the report said.

Only three of the current Guantanamo prisoners have been convicted of crimes and seven are currently facing charges, while the United States is weighing whether to file charges against 24 others.

But the cost savings of closing the remote Guantanamo detention facility could be considerable. The report said the United States spends more than $114 million a year to operate it, or about $686,747 per prisoner.

The Bureau of Prisons spent $94.87 a day per inmate at its high-security prisons last year, which works out to about $34,628 a year.

(Editing by Cynthia Osterman)