WASHINGTON (Reuters) - The United States is advancing toward the first transfer of a prisoner from the Guantanamo Bay detention center under U.S. President Donald Trump, the U.S. military told Reuters on Monday, a move that would repatriate the detainee to Saudi Arabia.

The transfer of 43-year-old Ahmed Muhammed Haza al-Darbi appeared to have stalled in February, when he became eligible but was not repatriated, as allowed under the terms of al-Darbi’s 2014 plea bargain agreement.

The U.S. military said at the time it was waiting for assurances from Saudi Arabia’s government to move forward on his departure.

On Monday, the Pentagon suggested that process was back on track. Navy Commander Sarah Higgins, a spokeswoman, said the Defense Department was “reviewing information received from Saudi Arabia regarding al-Darbi’s transfer.”

“The transfer process is moving forward,” Higgins said. “I have no further information on transfer timing.”

She did not elaborate.

Al-Darbi’s transfer would decrease the prisoner population at the detention center in Cuba to 40 from 41 - despite Trump’s campaign pledge to “to load it up with some bad dudes.”

The prison, which was 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.

U.S. officials have not ruled out again adding to the prisoner population and have acknowledged trouble repatriating Islamic State fighters being held by U.S.-backed forces in Syria, raising the possibility that Guantanamo Bay could be seen as a viable option in the future.

FILE PHOTO: A soldier stands guard in a tower overlooking Camp Delta at Guantanamo Bay naval base in a December 31, 2009 file photo provided by the US Navy. REUTERS/US Navy/Spc. Cody Black/Handout via Reuters

The news of progress in al-Darbi’s case came before a high-profile trip to Washington by Saudi Crown Prince Mohammed Bin Salman, his first visit to the United States as Saudi heir apparent.

The crown prince is due to meet Trump at the White House on Tuesday, in a visit that is expected to see both countries publicly emphasize their strong security ties, even as those ties come under intensifying scrutiny in Congress.

‘MESSAGE OF HOPE’

Al-Darbi became eligible for repatriation under the terms of his 2014 plea bargain agreement, in which he admitted to his role in a 2002 attack of against a French-flagged oil tanker off the coast of Yemen, and agreed to cooperate with U.S. authorities. He has been held at Guantanamo Bay for 15 years.

Ramzi Kassem, a law professor at the City University of New York who has been al-Darbi’s lead defense counsel since 2008, told Reuters that a Trump administration move to honor the 2014 agreement would be significant and would be noticed by other inmates.

“Mr. Darbi’s transfer to Saudi custody would send a qualified message of hope to other prisoners that leaving Guantanamo is possible,” Kassem said.

He said the message was qualified in part because “the U.S. government has no interest in prosecuting three quarters of the detainees left today,” which would make them ineligible for such agreements.

Critics of the U.S. military detention system say that 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.

Trump’s 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.

Trump signed an order in January to keep the Guantanamo Bay detention center open and hinted in his State of the Union address to Congress that Islamic State or al Qaeda fighters could be added to the prison population

Trump also asked the Pentagon to re-examine the U.S. military’s detention policy.

The U.S. 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.

In February, U.S. officials expressed concerns about the lack of a clear path on how to deal with foreign fighters for Islamic State who were detained by the U.S.-backed Syrian Democratic Forces.

U.S. officials say there are hundreds of foreign fighters and thousands of Syrian Islamic State militants in SDF hands.