One of the alleged September 11 attack planners held at Guantanamo Bay is in desperate need of medical care after allegedly being sodomised by a foreign object and denied treatment, a human rights group says.

Saudi national Mustapha al-Hawsawi - one of the "9/11 Five" accused of plotting the 2001 attacks that killed about 3,000 people - is suffering "painful and humiliating" rectal problems as a result of what Amnesty International said may have been torture.

The London-based rights group - in a letter to the Pentagon health affairs chief - demanded better medical care for the detainee.

Amnesty said Hawsawi's problems "may well be the result of torture by US government personnel: namely sodomy with a foreign object while in CIA custody".

Mustafa Ahmad al-Hawsawi [International Committee of the Red Cross via AP]

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 he told the AFP news agency that "all detainees are provided medical and dental care equivalent to that received by US service members" at Guantanamo Bay.

The letter, which was signed by the group's interim US executive director Margaret Huang, said "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".

Hawsawi has claimed in the past that he suffered damage to his rectum during aggressive CIA body cavity searches and has been denied surgery to correct it.

His lawyers have argued that the violence of the searches amounts to sodomy.

More than 14 years after the September 11 attacks, Hawsawi and his four co-defendants are still mired in endless military court proceedings.

Countless motions by the defence and the logistical headache of transporting the legal staff and other personnel for each hearing has prevented major progress.

Also among the "9/11 Five" is Khalid Sheikh Mohammed, who has publicly admitted to being the principal planner of the September 11 attacks.

Al-Hawsawi and four co-defendants are due in court next week for a pretrial hearing in their long-stalled case.

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Source: AFP