Lawyers for the five men accused of the September 11, 2001 attacks invoked the UN Convention against Torture on Tuesday as a way to bring up their clients' alleged mistreatment in secret prisons.

However, the harshest treatment, the use of simulating drowning and sleep deprivation, will be dealt with only in closed door sessions on orders of Judge James Pohl, because they involve classified information.

"That convention (against torture) gives certain rights" to the accused, Ruiz explained.

But "those rights do not exist, certainly not in front of this commission," he argued, emphasizing that his client is "living in a box in Guantanamo Bay, Cuba."

The lawyers had earlier protested against new violations in their "privileged" communications with their clients, alleging continuing searches of the inmates' legal mail in their cells, despite a judge's order forbidding it.

"You have the power to dismiss the death penalty or dismiss these charges because of the obstacles we face in this case," said Walter Ruiz, a lawyer for detainee Mustafa al-Hawsawi.

Sitting beside his fellow defendants, the self-proclaimed mastermind of the attacks, Khalid Sheikh Mohammed, was clad in a long, white tunic and a traditional headdress, and sported his usual thick beard dyed orange with fruit juice.

The five listened in silence as the defense lawyers asked the judge to allow testimony from international experts, including the former UN special rapporteur on torture, at the tribunal.

"Some aspects require some knowledge of international law," said James Connell, lawyer for Mohammed's nephew, Ali Abd al-Aziz Ali, also known as Ammar al-Baluchi, in arguing for the experts to be brought in.

But the US government said it would oppose bringing experts to the hearings, saying that "everyone should be able to argue whether the convention against torture is relevant in front of this commission."

And the judge emphasized he didn't have the power to "order somebody to leave the US to come to Cuba" to testify before the special military tribunal.

The accused face the death penalty if convicted of plotting the attacks on New York and Washington 12 years ago, which left nearly 3,000 people dead.

The preliminary hearings began in May 2012, but a date for the trial has yet to be set.