US military said Ahmed Muhammed Haza al-Darbi's transfer to Saudi Arabia reduced the detainee population to 40 from 41.
The United States announced on Wednesday the first transfer of a prisoner from the Guantanamo Bay detention centre under US President Donald Trump, lowering the prisoner population at a facility Trump has signalled he would like to repopulate.
The US military said Ahmed Muhammed Haza al-Darbi's transfer to Saudi Arabia reduced the detainee population to 40 from 41. He was the first prisoner to leave the US detention centre in Cuba since Jan. 19, 2017 - the day before Trump was inaugurated.
Reuters reported in March that al-Darbi's transfer was advancing.
Commander Sarah Higgins, a Pentagon spokeswoman, noted that the transfer was provided under the terms of a 2014 plea deal, which will allow al-Darbi to serve out the rest of his 13-year sentence there.
"The United States coordinated with the government of the Kingdom of Saudi Arabia to ensure the transfer took place in accordance with established standards for security and humane treatment," Higgins said.
The prison, opened by Republican President George W. Bush to hold terrorism suspects captured overseas after the Sept. 11, 2001, attacks came to symbolize harsh detention practices that opened the United States to accusations of torture.
US officials have not ruled out adding to the prisoner population again and have acknowledged trouble repatriating Islamic State fighters being held by US-backed forces in Syria, raising the possibility that Guantanamo Bay could be seen as a viable option in the future.
'BAD DUDES'
Trump's Democratic predecessor, Barack Obama, reduced the inmate population to 41 from 242, but fell short of fulfilling his promise to close the jail before leaving office last year.
In contrast, Trump pledged during his campaign to repopulate the prison, saying he wanted to "to load it up with some bad dudes."
Trump signed an order in January to keep the detention centre open and hinted in his State of the Union address to Congress this year that Islamic State or al Qaeda fighters could be added to the prison population.
He also asked the Pentagon to re-examine the US military's detention policy.
To that end, the Pentagon said on Wednesday it updated its guidance on criteria for transferring new detainees to Guantanamo Bay.
"This policy provides our warfighters guidance on nominating detainees for transfer to Guantanamo detention should that person present a continuing, significant threat to the security of the United States," said Higgins.
A White House National Security Council spokesman confirmed it had received the new detainee criteria.
"We have no further comment at this time," the spokesman said.
The US military has long struggled with what to do about prisoners of war in an open-ended battle against Islamist extremism, in which militants have come from all corners of the world to fight in places like Syria.
Defence Secretary Jim Mattis told reporters on Monday that there were well over 400 foreign fighters being held by US-backed forces in Syria. Although there was an effort to send those fighters back to their home countries, that was not always easy, he noted.
"In some cases, those countries (where the fighters came from) have stripped them of their citizenship," Mattis said.
"So they have a different view as far as what their status is today. So this is not simple."
Critics of the US military detention system say militants can be best prosecuted in civilian courts and have seized on the high costs of housing inmates at Guantanamo Bay as one argument why indefinite detention is misguided.
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