WASHINGTON - The transfer of prisoners out of Guantanamo Bay has ground to a halt amid a slow Pentagon approval process, causing deep frustration within the administration and raising doubts that President Barack Obama will be able to fulfill his campaign promise to close the prison for terrorism suspects at the US Navy base in Cuba.

A detainee sent back to his native Algeria in March is the only prisoner to have moved out this year, beyond the controversial exchange of five Taliban members in return for long-held captive US Army Sgt. Bowe Bergdahl, who was captured in Afghanistan.

The slow pace is the result of the law that gives Defense Secretary Chuck Hagel - not the commander in chief, Obama - the final authority to transfer any of the 149 terror suspects being held at Guantanamo. Pentagon officials say they must carefully consider the risks before signing off, given that others have returned to terrorism.

The White House has reminded the Pentagon that recidivism risks must be weighed against the danger to the United States in keeping Guantanamo open. Obama has said Guantanamo's continued operation hurts US standing overseas and is a recruitment tool for terrorists.

"The president would absolutely like to see more progress in our efforts to close Guantanamo," Obama counterterrorism adviser Lisa Monaco said. "He wants it closed. He's pushing his own team very hard, raising it weekly with me, with Secretary Hagel, with Secretary (of State John) Kerry. He also wants Congress to act to remove the restrictions in place that are making it even harder to move forward."

For years, Congress used its budget power to block Obama from making transfers. The president announced in May 2013 that he was appointing special envoys for Guantanamo closure at the State and Defense departments to move prisoners out "to the greatest extent possible." Congress responded by lifting some of the complicated restrictions for transfers, allowing them when Hagel determines steps have been taken to reduce the risk that detainees will re-engage in the fight.

"My name goes on that document, that's a big responsibility," Hagel said earlier this year. "I'm taking my time. I owe that to the American people, I owe that to the president."

Hagel was responding to a question about the months that had passed since Uruguay offered to take six detainees. Hagel eventually signed off, after a call from White House Chief of Staff Denis McDonough, according to administration officials. Meanwhile, controversy grew over the deal in Uruguay and officials there say it is now unlikely to proceed before the fall election.

Ian Moss, who works in the office of State Department envoy Clifford Sloan, said Uruguay and the US are "absolutely committed" to making the transfer, but he can't say when. "We are actively engaging a wide variety of governments and working diligently to transfer each of 79 detainees currently approved for transfer," Moss said.

Administration officials say the State Department has reached or is finalizing agreements with foreign governments to accept about two dozen detainees. An administration official said 11 of those deals are awaiting Hagel's signature, some for many months.

"Many countries are willing to help, but willingness is not everything," said Paul Lewis, the Pentagon's envoy for Guantanamo closure. He said Hagel is "absolutely committed to close Guantanamo," but under the law Pentagon officials have to closely examine security in the receiving country, including whether detainees who have been previously released have returned to the fight or could move over a porous border. "We're being careful and deliberative."

Administration officials described a viewpoint among some who work on detainee policy in the military that Guantanamo should remain open indefinitely rather than risk that a detainee will return to terrorism. Some detainees have been released and then recaptured, including June's arrest in Spain of a former prisoner accused of recruiting militants for the Islamic State group.

Administration officials say Obama national security adviser Susan Rice wrote to Hagel in May, laying out Obama's view that there's never zero risk in transferring a detainee but that assessment must be balanced against the risk of keeping the prison open.

"The president's expectation is that all detainees who have been determined to be eligible for transfer or release ... will be repatriated or resettled from Guantanamo Bay as quickly as possible, consistent with US national security interests," Rice's letter said, according to parts read to The Associated Press.

Transferring out approved detainees is only the first step to shuttering the prison established in the aftermath of the Sept. 11, 2001, terrorist attacks. A more complicated question looms over how to handle the most dangerous prisoners. Congress has prohibited Obama from bringing them to the United States for detention or trial, leaving them nowhere to be held other than Guantanamo.