WASHINGTON - The United States announced on Wednesday the first transfer of a prisoner from the Guantanamo Bay detention center under US President Donald Trump, reducing the prisoner population at a facility Trump has signaled 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 center 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, terrorist 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.

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 load it up with some bad dudes".

Trump signed an order in January to keep the detention center 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.

Reuters