The Pentagon on Monday announced that 15 men would be transferred from Guantánamo Bay to the United Arab Emirates, in the largest single detainee shuffle under President Barack Obama's administration.

The transfer means there are now just 61 people left in the U.S. military prison in Cuba. The 15 men include 12 Yemenis and three Afghans.

Amnesty International hoped the move indicated that the Obama administration would step up its efforts to close the controversial site.

"This is a powerful sign that President Obama is serious about closing Guantánamo before he leaves office. With these transfers, Guantánamo's population will be reduced by one-fifth," said Naureen Shah, Amnesty International USA's security and human rights program director.

"It is vital he keep the momentum. If President Obama fails to close Guantánamo, the next administration could fill it with new detainees and it could become permanent. It would be an extremely dangerous legacy of allowing people to be detained without charge, in an endless global war, practically until they die," Shah said.

Among those released is Zahir Hamdoun, a Yemeni citizen held at Guantánamo without charge since 2002, when he was 22 years old. The Center for Constitutional Rights (CCR), which represents Hamdoun, said Monday that until he was cleared for release in January by the Periodic Review Board (PRB), Hamdoun had been slated for indefinite detention.

Because the U.S. has a policy forbidding the repatriation of Yemeni citizens, Hamdoun has been made into a refugee, the CCR said.

"During a phone call before his release, Mr. Hamdoun said he felt happy and hopeful—a remarkable sentiment from a man who has lived through hell in Guantánamo and lost over 14 years of his life," said CCR senior staff attorney Pardiss Kebriaei.

The U.S. "must at least help ensure that Mr. Hamdoun can see his family—in particular his mother—without delay after his transfer, after depriving him of his loved ones for all these years. Even that much is not clear for the men just transferred," Kebriaei added. "Release must mean not only physical transfer from Guantánamo, but the restoration of these men's basic freedoms."

Amnesty also highlighted the case of an Afghan man named Obaidullah, who had been imprisoned without trial for 14 years and alleges that he was tortured, beaten, and threatened with sexual abuse while in U.S. custody.

"Many of the remaining detainees have been held without charge for a decade or more. Each detainee must either be charged and face trial in federal court, or be released to countries that will respect their human rights. As long as Guantánamo remains open, the U.S. risks making this ugly stain on its human rights record permanent," Shah said.

Obaidullah was featured in Amnesty's 2013 report, "I Am Fallen Into Darkness," which explained that he was just 19 years old when he was "taken from his home in eastern Afghanistan in the middle of the night by U.S. armed forces."

And CCR featured a letter written by Hamdoun to his attorney, published in the Guardian, that described his experience of imprisonment: "I have become a body without a soul. I breathe, eat and drink, but I don't belong to the world of living creatures. I rather belong to another world, a world that is buried in a grave called Guantánamo."