The Trump administration has made its first major move relating to prisoners at the military prison at Guantánamo Bay, Cuba, surprising observers by doing the exact opposite of the president’s proclaimed intention: instead of bringing in new detainees it has transferred an inmate off the island to Saudi Arabia.

The unexpected news came in a Pentagon announcement that it had completed the transfer of Ahmed al-Darbi, 43, a self-confessed al-Qaida member who pleaded guilty in 2014. According to McClatchy, the prisoner was picked up by a Saudi government plane on Tuesday night and taken to a detention center in the kingdom where he is likely to remain until he has finished his full 13-year sentence in 2027.

Ahmed al-Darbi, holding a photograph of his children, photographed inside Guantánamo. Photograph: AP

Al-Darbi’s removal reduces the total number of Guantánamo detainees to 40, down from a total of about 780 men who have been held at the controversial site since the beginning of the war in Afghanistan in 2002.

The transfer was all the more extraordinary in that it came on the same day that the Pentagon delivered what it described as an “updated policy” setting out the conditions for the transfer of new detainees to Guantánamo. James Mattis, the defense secretary, handed over the policy on Wednesday, saying that it would give “warfighters guidance on nominating detainees for transfer to Guantánamo detention should that person present a continuing, significant threat to the security of the United States”.

Trump made much of his intention to restock Guantánamo on the 2016 campaign trail, and was scathingly critical of his predecessor, Barack Obama’s, failed attempt to close the military detention camp. In January, Trump signed an executive order that instructed the Pentagon to keep the prison camp open and to produce within 90 days a policy review setting out how new detainees could be imported to the base.

Mattis delivered that review to the White House two days late. Though it has not been made public, reports suggest that it is vaguely worded and changes existing policy relatively little.

So far, no new detainees have arrived at Guantánamo. But the Trump administration has followed the president’s ideological bent and dramatically slowed up procedures relating to the civil rights of the existing inmates, and has also shuttered Obama’s Office for Guantanamo Closure.

That in turn delayed al-Darbi’s release by several months. In a statement reported by the New York Times, al-Darbi made an impassioned attack on the continuing existence of the camp outside the normal checks and balances of the judicial system.

He said: “My words will not do justice to what I lived through in these years and to the men I leave behind in prison. No one should remain at Guantánamo without a trial. There is no justice in that.”

Al-Darbi was captured in 2002 in Azerbaijan. He admitted to being involved in al-Qaida attacks by boat in the Arabian Peninsula, and also acted as a government witness in other cases.

In a statement to the New York Times, al-Darbi’s lawyer, Ramzi Kassem, a New York law professor, warned that his client’s transfer out of Guantánamo might be the last move of its kind given Trump’s strident posture. “This is the first prisoner transfer under Trump, but it may also be the last unless the courts meaningfully check the president’s claimed power to imprison men without charge for as long as he pleases.”

Clive Stafford Smith, founder of the civil rights group Reprieve which also represents Guantánamo detainees, said: “While it is always welcome when someone leaves Guantánamo, this just shows that the only way to get out now is to plead guilty, whether it is coerced and involuntary or not. The president should be focused on immediately releasing the five men who have already been cleared by six federal agencies and then giving the remaining 35 detainees a fair trial or setting them free.”