The razor wire-topped fence and the watch tower of "Camp 6" detention facility at the US Naval Station in Guantanamo Bay, Cuba. (Mladen Antonov/AFP/Getty Images)

The Obama administration has moved five additional prisoners out of the Guantanamo Bay detention center, sending four Yemeni inmates to the Persian Gulf sultanate of Oman and a fifth to Estonia, the Pentagon said Wednesday.

The latest transfers leave 122 detainees at the military prison in Cuba and underscore President Obama’s determination to press forward with a long-standing effort to close it.

The detainees sent to Oman are Al Khadr Abdallah Muhammad al Yafi, Fadel Hussein Saleh Hentif, Abd al-Rahman Abdullah Au Shabati and Mohammed Ahmed Salam, the Pentagon said in a statement. Akhmed Abdul Qadir was sent to Estonia.

According to military documents made public by WikiLeaks, the United States has suspected all five detainees, imprisoned since 2001 or 2002, of links to al-Qaeda. But officials have said that much of the information collected in such files ultimately proved to be inaccurate.

Wednesday’s transfers are significant in light of the dilemma facing U.S. officials because of Guantanamo’s population of Yemeni nationals. Yemenis make up the largest single national group remaining at the highly fortified facility, but the United States has been unwilling to send prisoners to Yemen because of instability there and concerns that the government would be unable to properly monitor them.

Pentagon officials have said they were hopeful that Persian Gulf nations, many of them U.S. allies in the fight against extremism, would accept Yemenis from Guantanamo. In late December, three Yemenis were sent to Kazakhstan as part of an inmate transfer.

“We take our obligation to assess the security risk of detainees seriously prior to transfers,” Paul Lewis, a senior Pentagon official involved in the effort to close the Guantanamo Bay detention center, said in a statement. “As a result, more than 90 percent of detainees transferred during the Obama administration live quietly around the world.”

Senior U.S. officials say they hope to accelerate transfers in 2015 of the dozens of prisoners already cleared for release from Guantanamo and to complete reviews that could clear others to be moved. Obama is taking a more active role in the renewed effort to close the facility, pressing foreign heads of state to accept detainees and making a fiscal case for shutting the prison, which costs $400 million to $500 million a year to operate.

But opposition remains substantial in Congress, especially among lawmakers who object to bringing Guantanamo detainees to the United States for trial or further detention. Others express concern about detainees who have returned to militancy after being released from the prison.

U.S. officials say they hope to win over some naysayers in Congress as the prison population shrinks. Even after all detainees who can be moved to other countries are gone, a smaller number are expected to remain — perhaps 20 to 30 — who either cannot be tried because of insufficient or problematic evidence or are deemed too dangerous to be released.

Ten prisoners are at some stage of a military trial process, including five linked to the Sept. 11, 2001, attacks.

Julie Tate contributed to this report.