Amnesty International said a Guantanamo detainee accused of taking part in the 9/11 terror attacks may have been sodomized while in CIA custody and called on US authorities to provide medical care.

In a letter made public on Wednesday, Amnesty International said Mustafa al-Hawsawi is suffering "painful and humiliating" rectal problems, which "may well be the result of torture by US government personnel: namely sodomy with a foreign object while in CIA custody."

The rights group called on the Pentagon health affairs chief to provide better care for the Saudi national, who is one of the "9/11 Five" accused of plotting attacks that killed thousands of people on September 11, 2001.

The letter, which became public one week after it was written, added that Hawsawi's lawyers determined he had "received only minimal symptomatic treatment, such as over-the-counter pain medicine, for many of his conditions."

Pentagon spokesman Gary Ross declined to comment on the specifics of Hawsawi's case, but said that "all detainees are provided medical and dental care equivalent to that received by US service members" at Guantanamo Bay.

The letter, signed by Amnesty's interim US executive director Margaret Huang, said that "failure to ensure appropriate and continuing medical treatment and rehabilitative care for those deprived of their liberty, including those who have been subjected to torture or other ill-treatment, contravenes US international legal obligations."

— Close Guantanamo (@closegitmo) February 9, 2016

More than fourteen years after the 9/11 attacks, the cases against Hawsawi and his four co-defendants have yet to make significant progress.

President Barack Obama has failed to deliver on his 2012 campaign promise to close the US detention center in Guantanamo Bay, Cuba, and it looks likely he will be unable to do so before leaving the White House.