Saudi citizen Ahmed Mohammed Ahmed Haza al-Darbi has cooperated with the government since entering his plea four years ago, and prosecutors agreed he should be sent to a Saudi rehabilitation center some time after February 20.

If transferred, Darbi would be the first Guantanamo inmate to be released since President Donald Trump took office more than a year ago.

"Upon completion of all conditions, he may request to serve out any remaining portion of his sentence to confinement in the Kingdom of Saudi Arabia," Pentagon spokeswoman Commander Sarah Higgins said in a statement.

"These conditions, which are not in al-Darbi's control, have not been finalized. He will continue to remain at Guantanamo until all details have been concluded."

Darbi, who is the brother-in-law of one of the September 11, 2001 plane hijackers, admitted to planning, aiding and supporting an attack on the MV Limburg which killed a Bulgarian sailor, injured a dozen and caused a large oil spill in the Gulf of Aden.

According to a transcript of an October hearing, the government recommend Darbi be sentenced to 13 years in prison, to include time served since his 2002 confinement at Guantanamo.

"The accused took a number of actions to ensure the successful completion of the attack on the Limburg, including providing or obtaining visas for the terrorists that actually took part in the attack, providing them housing, purchasing equipment," prosecutor Captain Matthew Hracho said.

As part of his plea, Darbi has provided evidence against Saudi Guantanamo detainee Abd al-Rahim al-Nashiri, who faces the death penalty on charges he masterminded the MV Limburg attack and the 2000 attack against the USS Cole in Yemen that left 17 dead.

Darbi's lawyer did not immediately respond to a request for comment.

A total of 41 inmates remain at Guantanamo but Trump has vowed to send more detainees to the notorious prison on a US naval base on Cuba.

Of the 41, five inmates apart from Darbi were cleared for transfer under president Barack Obama, but efforts to fly them out before Trump took office failed at the last minute.