According to a State Department official, who spoke on the condition of anonymity, 12 of the men transferred to the United Arab Emirates (UAE) were from Yemen, while three are Afghans. The Pentagon had previously struggled to find a third country to send Yemeni detainees to, on account of the civil war in their home nation.

Under reviews conducted by a task-force made up of six US security agencies, six of the 15 transferees were deemed to no longer present a security risk, while nine others were recommended for transfer, based on a decision that their detention was no longer necessary to limit whatever risks they continued to present.

"The United States is grateful to the government of the United Arab Emirates for its humanitarian gesture and willingness to support ongoing US efforts to close [Guantanamo]," the Pentagon said in a statement. The release of the 15 to the UAE follows last month's transfer of a Tajik and a Yemeni to Serbian custody, and another prisoner to Italy.

The UAE had previously accepted a number of Guantanamo inmates.

The latest transfers bring the remaining population of the detention center down to 61. Since the September 11, 2001 attacks, about 780 inmates have been housed in the US military-run facility in Guantanamo Bay on the island of Cuba.

Mixed reactions

Amnesty International welcomed the announcement as a sign President Barack Obama was working towards closing the controversial facility before he leaves office in January 2017.

"It's a significant repudiation of the idea that Guantanamo is going to be open for business for the indefinite future," Naureen Shah, Amnesty International USA's security and human rights program director, told the AFP news agency.

However, not everyone welcomed the news of the transfers. Republican Representative Ed Royce, chairman of the House Foreign Affairs Committee, said in a statement that the Obama administration was "doubling down on policies that put American lives at risk" in a race to close the controversial facility.

"Once again, hardened terrorists are being released to foreign countries where they will be a threat," he said in a statement.

Republican Senator Kelly Ayotte had also recently renewed calls to keep Guantanamo open, after publishing a report on 107 current and former detainees that according to her highlighted their terrorist pasts.

"The more Americans understand about the terrorist activities and affiliations of these detainees, the more they will oppose the administration's terribly misguided plans to release them," she said.

Special envoy for Guantanamo closure Lee Wolosky replied that the continued operation of the detention facility actually weakened the US' "national security by draining resources, damaging our relationships with key allies and partners, and emboldening violent extremists."

"The support of our friends and allies - like the UAE - is critical to our achieving this shared goal."

Obama's ambitious plan

US President Barack Obama urgently wants to close the facility before he leaves office at the start of next year but has been continually thwarted by Republican lawmakers. When Obama took office, there were 242 detainees at Guantanamo, of which he has transferred almost three quarters to other countries or facilities.

The United States has in recent months accelerated the rate at which detainees were released from the facility. There are still another 19 inmates remaining at Guantanamo who have been cleared for transfer.

Obama's plan for closing down the facility also calls for bringing several dozen remaining prisoners to maximum-security prisons in the United States. US laws bar such transfers to the mainland. President Obama, however, has not ruled out pushing through the transfers by executive action.

The Office of the Director of National Intelligence says that five percent of Guantanamo prisoners released since Obama took office have re-engaged in militant activities and that an additional 8 percent were suspected of doing so. The numbers compare with 21 percent confirmed and 14 percent suspected during the Bush administration.

ss/kl (AFP, AP, Reuters, dpa)