"The Department of Defense announced today the transfer of Abdul Latif Nasir from the detention facility at Guantanamo Bay to the Kingdom of Morocco," a Pentagon statement said, adding that 39 prisoners remain in the controversial facility.

Nasir was never charged with any crime.

His transfer out of Guantanamo was first recommended by the Obama administration "subject to security and humane treatment assurances," according to the Pentagon, but had not taken place by the time Trump took office in 2017.

Former president Barack Obama failed in his bid to close the prison at a US military base in Cuba, which became a symbol of excesses in the "war on terror" launched after the September 11, 2001 attacks.

The Republican Party blocked Obama's plans to shut the prison by 2009 by restricting the ability of the United States to move prisoners from Guantanamo to the US mainland.

Obama's successor Trump mused about sending more suspects to Guantanamo Bay but effectively kept it in place in the same form.

President Joe Biden's White House in February launched a study into how to close the prison but has been careful not to overpromise after the failure of Obama's vow.

The Biden administration "is dedicated to following a deliberate and thorough process focused on responsibly reducing the detainee population of the Guantanamo facility while also safeguarding the security of the United States and its allies," the State Department said in a statement Monday.

It praised Morocco's role, saying that the kingdom's "leadership in facilitating Nasir's repatriation, alongside its past willingness to return its foreign terrorist fighters from northeast Syria, should encourage other nations to repatriate their citizens who have traveled to fight for terrorist organizations abroad."

Tarred by accusations of extra-legal imprisonment, denial of rights and torture, Guantanamo was opened in 2002 for suspects captured around the world on the understanding that they would not be entitled to the constitutional right to due process guaranteed on US soil.