The United States has transferred five men from its Guantanamo Bay prison, the Pentagon said Wednesday, in a renewed push by President Barack Obama toward closing the controversial jail.

Four of the inmates were sent to Oman, while one was sent to Estonia, according to Pentagon statements.

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

The four men 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 final inmate, Akhmed Abdul Qadir, was transferred to Estonia.

The transfer of the five men leaves 122 inmates at the remote prison, which is located at a US naval base in southeastern Cuba.

It is the first prisoner transfer of the year, after a total 28 inmates were moved in 2014.

In December, six Guantanamo inmates were transferred to Uruguay in South America after spending 12 years in the prison without being charged.

- Uruguay took six -

Uruguay's leftist President Jose Mujica had announced in March that the South American country would take in the inmates on "humanitarian grounds", in a bid to help Obama fulfil his long-delayed promise to close the prison set up in the wake of the 9/11 attacks.

One was Syrian national Jihad Diyab, 43, who had staged a hunger strike and requested a US court to order prison officials to stop force-feeding him.

The others were three other Syrians, a Palestinian and a Tunisian. All were in their 30s and 40s and among the first detainees sent to Guantanamo in 2002.

They had been cleared for release, but the US ruled they could not be sent to their home countries for security reasons.

A total of 779 prisoners have been held at Guantanamo in the nearly 13 years since the detention center was set up at the US naval base in the southeastern corner of Cuba.

Thwarted by Congress in his effort to close the prison, Obama has had to rely on a handful of countries that have agreed to accept detainees.

Uruguay is the first South American country to do so.

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.

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.