The population of the US prison at Guantanamo Bay in Cuba has dropped below 100 for the first time since it opened in 2002, with the transfer of 10 Yemeni men to Oman.
The releases reflect progress by President Barack Obama towards achieving his goal of closing the prison, American officials say.
The State Department has thanked Oman for taking the prisoners.
The country is temporarily hosting them until a permanent placement is found.
Officials in Oman say they arrived on Thursday and are likely to remain there until conditions improve in war-hit Yemen.
This is the largest single transfer of prisoners from Guantanamo, correspondents say, and is a fillip for the president who began his first term of office in 2009 by promising speedily to close the centre.
The prison has been repeatedly criticised by human rights activists.
All of those most recently transferred were held for more than 10 years without charge or trial.
They are part of a series of releases promised by the president for this January as his administration makes plans for the permanent closure of the facility.
Other detainees were also moved earlier this month including two Yemenis who were sent to Ghana.
The prisoners' rights campaign group Reprieve says that among those released on Thursday is Samir Naji Moqbel who was held for 14 years without charge in Guantanamo.
He was taken to the outdoor pens of Camp X-Ray and subjected to severe physical abuse, Reprieve says, before being cleared for release in 2009 with the unanimous agreement of six US federal agencies, including the CIA and the FBI.
In 2013, Mr Moqbel wrote about a mass hunger strike at the camp in the New York Times.
Reprieve says that many other hunger-strikers in Guantanamo Bay have been violently force fed.
Mr Obama has faced opposition over the releases in the Republican-led Congress, where lawmakers especially object to the transfer of prisoners to other facilities within the US.
The Gulf Arab country of Oman is a close US ally and has previously accepted other groups of Guantanamo prisoners.
It is estimated that there are now about 93 prisoners remaining at Guantanamo - the lowest total since 2002 when President George W Bush opened the facility to accommodate foreign terrorism suspects after the 9/11 attacks on the US.
A total of 780 men have been held at Guantanamo Bay since 2002, the vast majority without charge or criminal trial.
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