MIAMI/MUSCAT (Reuters) - Ten Yemeni men held at the Guantanamo Bay U.S. military prison were sent to Oman on Thursday, bringing the detainee population below the symbolically important milestone of 100 as President Barack Obama steps up efforts to close the facility before he leaves office.

Their transfer to the Gulf Arab state marked the largest group of prisoners shipped out of the detention center at the U.S. naval base in Cuba since Obama began his presidency in 2009 pledging to quickly shutter a prison that has drawn international condemnation.

The Yemenis, all held for more than a decade without charge or trial, were part of a wave of releases that the Obama administration signaled would take place early this year as it prepares to give Congress a plan for closing the facility. Four other detainees were moved out already this month.

“This is a significant milestone on the road to closing the facility,” White House spokesman Josh Earnest told reporters aboard Air Force One as Obama returned to Washington from Louisiana.

“We’ll continue to work diligently to reduce the prison population through safe and responsible detainee transfers to close the detention facility,” Earnest said.

Obama, whose term in office ends in January 2017, has vowed to push ahead with his efforts but faces opposition in the Republican-led Congress. Lawmakers have created obstacles to moving any Guantanamo prisoners to facilities in the United States.

In Muscat, an Omani official was cited by the state news agency as saying the Yemenis had arrived and would remain there for humanitarian reasons until conditions in Yemen, gripped by civil war, allow them to be sent home. Oman, a close U.S. ally, had accepted earlier groups of Guantanamo prisoners.

In this photo reviewed by a U.S. Department of Defense official, a Guantanamo detainee holds onto a fence inside the Camp 6 high-security detention facility at Guantanamo Bay U.S. Naval Base April 27, 2010. REUTERS/Michelle Shephard/Pool

U.S. Defense Secretary Ash Carter said the transfer followed a “deliberate and careful review.”

“We completed the transfer of 10 Yemenis - roughly 10 percent, that is, of the total remaining Gitmo population – to the government of Oman,” Carter told an audience at the U.S. military’s Southern Command, which oversees the military detention facility.

‘EXPENSIVE, UNNECESSARY’

The 93 prisoners remaining at Guantanamo mark the lowest number since 2002, shortly after then-President George W. Bush opened the facility to house foreign terrorism suspects after the Sept. 11, 2001, attacks on the United States.

Obama administration officials have said they will focus on repatriating or resettling the 34 Guantanamo prisoners, most of them Yemenis, cleared for release long ago by U.S. authorities.

The United States has ruled out sending the Yemenis home due to Yemen’s chaotic security situation.

Obama campaigned for the presidency in 2008 vowing to close the Guantanamo prison. In his final State of the Union address on Tuesday, he again urged Congress to help him achieve that goal.

“It’s expensive, it’s unnecessary and it only serves as a recruitment brochure for our enemies,” Obama said of the prison.

The White House has not ruled out that Obama could use executive powers to shut the prison, bypassing Congress. Some lawmakers have vowed legal action if he does that.

Carter said he had proposed to Obama establishing an alternative location that would bring some detainees - those deemed too dangerous to be transferred - “to an appropriate, secure location in the United States.”

“Congress has indicated a willingness to consider such a proposal,” Carter said.

A number of lawmakers, mostly Republican but also some Democrats, have made clear they would resist moving the inmates into U.S. prisons, even maximum security facilities where other terrorism suspects are being held.

The transfers “represent a thinly veiled attempt to undercut the will of Congress and would further endanger the American people,” U.S. Senator Kelly Ayotte, a New Hampshire Republican, said in a statement issued before the Yemenis were flown out.

The Pentagon identified the released detainees as: Fahed Abdullah Ahmad Ghazi, Samir Naji al-Hasan Muqbil, Adham Mohamed Ali Awad, Mukhtar Yahya Naji al-Warafi, Abu Bakr Ibn Muhammad al-Ahdal, Muhammad Salih Husayn al-Shaykh, Muhammad Said Salim Bin Salman, Said Muhammad Salih Hatim, Umar Said Salim al-Dini and Fahmi Abdallah Ahmad Ubadi al-Tulaqi.