Despite significant opposition from a group of Republican senators, the Department of Defense announced yesterday that five detainees would be transferred from the Guantanamo Bay military prison in Cuba to Oman and Estonia.

“The detainees that remain at Guantanamo are the worst of the worst, and their continued release will only further damage our national security,” says @SenatorBurr

Four of the detainees have been held at Guantanamo Bay for 13 years and will be transferred to Oman. They are: Al Khadr Abdallah Muhammad Al Yafi, Fadel Hussein Saleh Hentif, Abd Al-Rahman Abdullah Au Shabati, and Mohammed Ahmed Salam.

The fifth, Akmed Abdul Qadei, was held for 12 years and seven months and will be transferred to Estonia.

None of the five detainees was ever charged with a crime and all had been cleared for transfer for years.

Their release comes as President Obama works to empty the military prison. However, many Republicans disagree with the president’s plan to do so.

Yesterday, four senators—Sens. Kelly Ayotte of New Hampshire, Richard Burr of North Carolina, Lindsey Graham of South Carolina and John McCain of Arizona—introduced legislation to prohibit the release of high- and medium-risk detainees for two years. Their bill would also place a moratorium on the release of detainees to Yemen, where al Qaeda in the Arabian Peninsula is headquartered.

“The detainees that remain at Guantanamo are the worst of the worst, and their continued release will only further damage our national security,” Burr said in a statement. “Safety is my utmost concern, and a two-year prohibition on transferring the detainees with the highest risk should give Congress and the administration time to develop a long-term plan for keeping this country safe.”

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A senior leader of an al Qaeda affiliate in the Middle Eastern country has claimed responsibility.

Obama has authorized the release of 33 prisoners over the past year, bringing the total number of detainees housed at the military prison in Cuba to 122. In 2003, Guantanamo Bay housed 680.

The president promised to close the military prison during his 2008 campaign. He moved swiftly to do so during his first year in office, with 2009 marking the second highest year for releases. Last year marked the highest.

During the last two years of his presidency, Obama plans to lessen the prison population to between 60 and 80 detainees. The group Human Rights Watch estimates it costs the United States $3 million per year for each detainee.