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Nine detainees from the Guantanamo Bay detention center have been transferred to the government of Saudi Arabia, the Pentagon announced Saturday. Department of Defense officials told NBC News all nine are from Yemen.

This image reviewed by the US military shows the guard tower of "Camp Six" detention facility of the Joint Detention Group at the US Naval Station in Guantanamo Bay, Cuba, on January 19, 2012. JIM WATSON / AFP - Getty Images file

Among the nine being transferred is Tariq Ali Abdullah Ba Odah, who has been approved for transfer since 2009 and has been on a hunger strike since 2007. As of July 2015, he weighed 74 pounds and was regularly force-fed.

Related: Americans 'Died Because of' Released Gitmo Detainees: Pentagon Official

The Guantanamo Review Task Force conducted a "comprehensive review" of the detainees and approved them for transfer after examining "a number of factors, including security issues," according to a Pentagon news release.

"The United States is grateful to the government of the Kingdom of Saudi Arabia for its humanitarian gesture and willingness to support ongoing U.S. efforts to close the Guantanamo Bay detention facility," the release said. "The United States coordinated with government of the Kingdom of Saudi Arabia to ensure these transfers took place consistent with appropriate security and humane treatment measures."

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The move takes the total number of detainees at the U.S. military prison down to 80, with 26 approved to be moved out of the facility, which is located on Cuban territory held by America.

It comes ahead of a visit by Obama to the Saudi Arabian capital of Riyadh next week to meet with Gulf leaders to discuss the threat posed by ISIS.

According to U.S. officials, the Saudis agreed to accept the detainees after lengthy negotiations, marking a potential turning point in the U.S.'s often fraught relationship with Saudi Arabia.

Related: Americans 'Died Because of' Released Gitmo Detainees: Pentagon Official

Obama has long vowed to close Guantanamo. Late last month, Paul Lewis , the Defense Department’s Special Envoy for Guantanamo Detention Closure, told a House Foreign Affairs Committee hearing that Americans "have died because of" detainees released from the detention facility, fueling criticism for Obama's plan.

Even with this transfer, the majority of those left at Guantanamo are from Yemen. They are also the majority of those left who are approved for transfer: 21 of the 26 detainees.

Part of the reason so many Yemenis remain is due to a congressional ban on repatriating anyone back to Yemen amid the unstable situation there. The United States is also not allowed to transfer detainees to Libya, Somalia and Syria.

According to one official, multiple levels of the Obama administration have been lobbying Saudis to accept more detainees, especially Yemeni ones, many of whom have family in Saudi Arabia. That is the case with Ba Odah.

According to the same official, the “intense engagement accompanied by changing dynamics within the Saudi establishment created an environment to get this done.”

The official said the U.S. has not promised Saudi Arabia anything in return for taking this set of detainees. The men will have to go through the Saudi rehabilitation program before being allowed to re-enter society in that country.

The State Department expects to complete the transfer of the 26 detainees who are currently approved to leave the detention camp by this summer.

Late Friday night, a Yemeni human rights activist started to tweet about the transfer of nine Yemenis from Guantanamo Bay to Saudi Arabia. Partly out of caution for the military personnel involved in the transfer, the Department of Defense did not announce it until Saturday afternoon.