The United States has released five Yemeni inmates from Guantanamo Bay prison, at least six years after they were cleared for release from the controversial jail in Cuba.

Four of the inmates were sent to Oman, while one was sent to Estonia, the Pentagon said in a statement on Wednesday - the first time either nation has accepted Guantanamo prisoners for resettlement.

The men who are in their 30s and 40s, including one who was 17 when he was sent to Guantanamo, had been cleared for release in 2009, but the US balked at repatriating the prisoners back to Yemen.

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US officials said "a comprehensive review" of the cases was conducted by several agencies before the men were moved, and all were "unanimously approved for transfer".

The recent releases have angered some members of Congress, who have argued that Guantanamo is necessary to detain terrorism suspects.

"Now is not the time to be emptying Guantanamo," Senator Kelly Ayotte said at a news conference during which she warned of fresh terrorist threats.

Republican senators on Tuesday proposed restrictions that would bar transfers to Yemen for two years and suspend the transfer of men previously classified as high-risk or medium-risk.

The transfer of the five men leaves 122 inmates at the remote prison, more than 13 years after it opened and seven years after President Barack Obama promised to close it.

Obama's envoy overseeing the release of Guantanamo inmates, Cliff Sloan, resigned in December after reportedly becoming frustrated at how long it took the Pentagon to approve transfers of detainees.

The prison was set up to hold alleged terror suspects after the September 11, 2001, attacks, but human rights groups have condemned the jail as a "legal black hole", where inmates languish for years without being tried in court.

Source: Agencies