Five men from Yemen have been freed from the US Navy base at Guantanamo Bay, Cuba, after more than a dozen years of captivity and sent to Estonia and Oman for resettlement.
It is the latest in a wave of releases that have alarmed political opponents of closing the detention centre housing terror suspects.
Four of the men went to Oman and the other to Estonia, the Pentagon said - the first time either nation accepted Guantanamo prisoners for resettlement.
The men had been cleared for release since at least 2009 but the US has baulked at repatriating Guantanamo prisoners back to Yemen, where the government is battling an al Qaida uprising.
All five were captured in Pakistan and detained by the US as suspected al Qaida fighters. US officials later determined it was no longer necessary to detain them but have struggled to find other countries willing to take them in. The men are in their 30s and 40s, including one who was 17 when he was sent to Guantanamo.
President Barack Obama came into office pledging to close the detention centre on the base but was blocked by Congress, which barred transferring any prisoner to the US for any reason and imposed restrictions on sending them elsewhere.
Congress eased the restrictions on transfers abroad in December 2013 and the Obama administration has stepped up the releases in recent months.
There are now 122 prisoners at Guantanamo, including 54 who have been approved for transfer. Of those cleared to leave, 47 are Yemeni and will likely have to be resettled in other countries, given the security situation in their homeland.
"We are committed to closing the detention facility. That's our goal and we are working towards that goal," said Ian Moss, a spokesman for the US State Department on Guantanamo issues.
The recent releases have angered some members of Congress, who have argued that Guantanamo is necessary to detain terrorism suspects. Republican senators proposed restrictions on Tuesday that would bar transfers to Yemen for two years and suspend the transfer of men previously classified as high or medium-risk.
"Now is not the time to be emptying Guantanamo," Senator Kelly Ayottee said at a news conference during which she warned of fresh terrorist threats.