The watch tower of "Camp 6" detention facility at the U.S. Naval Station in Guantanamo Bay, Cuba. (Mladen Antonov/AFP/Getty Images)

The U.S. military has sent five detainees from the prison at Guantanamo Bay to Kazakhstan, the Pentagon announced late Tuesday, capping a year of intensified efforts by the Obama administration to shut down the detention center.

The transfer of the prisoners, three Yemenis and two Tunisians, to the central Asian nation brings the number of detainees moved out of Guantanamo this year to 28.

“The United States coordinated with the Government of Kazakhstan to ensure these transfers took place consistent with appropriate security and humane treatment measures,” the Pentagon said in a statement.

Officials are preparing to accelerate transfers in 2015 as President Obama attempts to make good on one of his key national security goals: closing the prison that became a global symbol of the excesses of the U.S. response to the Sept. 11, 2001, terrorist attacks.

Already in December, the Pentagon had sent four detainees home to Afghanistan and moved six prisoners to Uruguay. Detainees have also been transferred this year to Slovakia, Georgia, Algeria, Kuwait and Saudi Arabia.

The Pentagon identified the Yemeni detainees as Asim Thabit Abdullah al-Khalaqi, Muhammad Ali Husayn Khanayna and Sabri Muhammad Ibrahim al-Qurashi.

Kazakhstan’s decision to accept those prisoners, in particular, is a welcome development after years in which the Obama administration was unable to repatriate a large number of Yemeni prisoners because of fears that they would escape or be set free in Yemen. After Tuesday’s transfer, 81 Yemenis remain at Guantanamo, the largest single group among the 127 detainees still there.

Earlier this year, four Yemenis were moved to Georgia and Slovakia.

One of the Yemeni prisoners taken to Kazakhstan, Khalaqi, is said to have fought with al-Qaeda during the battle for Tora Bora in Afghanistan, according to U.S. military documents made public by WikiLeaks. He was captured in Pakistan in late 2001 and handed over to the United States.

The other two Yemenis were also believed to have been members of al-Qaeda, according to the documents, which are dated 2007 and 2008. None of the three were charged with a crime, and they were later deemed to pose a low enough threat to be released.

The leaked military documents describe one of the Tunisians, named Adel al-Hakeemy, as a military adviser to Osama bin Laden who battled U.S. and allied forces at Tora Bora and also fought in the Balkans.

The other Tunisian, named Abdullah Bin Ali al-Lufti , has suffered from extensive medical conditions, including chronic heart problems. He also received treatment for anxiety and depression while at Guantanamo.

A U.S. defense official said the prisoners will be “resettled” in Kazakhstan, a term the Pentagon uses when detainees are set free in a new country but remain subject to some level of monitoring by the host government. Typically, the released detainees are prohibited from leaving the host country for one or two years.

U.S. officials are now in talks with a range of countries that they hope will agree to take in some of the other 59 inmates cleared to be moved out of Guantanamo.

Julie Tate contributed to this report.

An earlier version of this story misstated the number of detainees transferred to Uruguay in December.