Thirteen years after terror suspects first arrived in Guantanamo, the United States is sending inmates out at a faster rate than ever before -- but hurdles remain in the effort to shutter it entirely.

In 2014, 28 detainees were freed in "the largest single-year reduction in detainee population" since 2009, said Paul Lewis, the US official charged with closing the Guantanamo detention facility.

As the facility enters its 14th year Sunday, Lewis told AFP the government is striving to "maintain momentum" of its prisoner releases.

"The road to closing Guantanamo is clear and well lit," wrote his former counterpart from the State Department Cliff Sloan in an editorial in the New York Times.

But Noor Mir from Amnesty International USA, is calling on the Obama administration to work faster.

"As we count a grim thirteenth year since Guantanamo opened, dozens of men continue to languish there with no idea when or even if their detention will end," he said.

The first detainees arrived at the US prison camp in Cuba on January 11, 2002 when, some four months after the attacks of September 11, 2001, president George W. Bush locked in open-air cages some of the "worst of the worst" in the country's war on terror.

Today, 127 Guantanamo inmates remain, down from some 680 prisoners held in 2003. The prison population could fall below 100 within two months, with the aim of cutting the population by half before the end of the year.

However, President Barack Obama's administration has been hampered by Republicans in Congress who oppose any transfer detainees to US soil, even for trial.

- Next transfer -

Of the prisoners remaining are 83 Yemenis. They pose the biggest problem for Guantanamo's closure, due to the volatile political situation in Yemen.

The first step for Obama, who has reiterated his intention to close the prison facility, is to find host countries for the 59 Yemenis cleared for release.

"We're trying to transfer those 59 as soon as possible, hopefully this year," a senior defense official told AFP.

"It's on a case by case, individual basis," the official said on the condition of anonymity.

The official said the US is focusing on finding destinations in South America and then in the Middle East and Europe.

Next week, five Yemenis are to be transferred to two unidentified countries. So far, nearly two dozen countries have accepted Guantanamo detainees originally from somewhere else.

US authorities seek countries that will take the prisoners, treat them well and monitor them.

The prisoners, who have never been convicted of a crime, are not intended to be treated as criminals after their release, but they must stay in the host country for at least two years.

The next step for the administration would then be the 10 "high value" prisoners who await prosecution.

Among them is self-proclaimed September 11 mastermind Khalid Sheikh Mohammed and his four co-defendants, who await a military trial.

The trial has not yet begun, dimming the prospect of closing Guantanamo in the near future.

Finally, to close the facility any time soon, the administration may need to expedite the review of each of the remaining 58 prisoners who have not been charged or approved for transfer.

Not all are expected to be released from prison, as some are committed jihadists, according to the government.

But estimates say the number of prisoners could be reduced to 40.

Obama could then turn to Congress to obtain permission to transfer the remaining detainees to high security US prisons, where the cost would be significantly less than the $3 million per inmate spent annually in Guantanamo.

"If he can't bring these detainees to the US, he will have to keep them in Guantanamo," said lawyer David Remes who represents 18 Yemeni detainees.

"I don't see Congress ever relenting and letting him bring them to the US, so Guantanamo will remain open," Remes said.