Lawyers for the five Guantanamo Bay prisoners charged over the September 11 terror attacks have challenged rules for handling secret evidence that they say prevent them from seeking an investigation into CIA torture claims.

In a pre-trial hearing at the US base in Cuba, lawyers sought to persuade a military judge that courtroom rules for handling classified evidence are so restrictive they violate the Convention Against Torture, a global treaty ratified by the US in 1994.

The lawyers argued that the rules prevent the defendants from disclosing details of their harsh treatment in the CIA's network of overseas prisons.

The five men facing trial by military commission were subjected to treatment that their lawyers say amounted to torture before they were taken to Guantanamo in September 2006.

They may want to pursue some form of international complaint against the US government, but cannot because they and their lawyers are not allowed to even discuss with anyone what happened to them in CIA custody.

Army major Jason Wright, a military-appointed lawyer for lead defendant Khalid Sheikh Mohammed, said the judge should either provide a mechanism to pursue a claim - either through his native Pakistan, the United Nations or another country - or dismiss the charges.

"You cannot use state secrets to classify the observations and secrets of someone who was subjected to torture," he said.

Mohammed and his four co-defendants face charges that include terrorism, hijacking and nearly 3,000 counts of murder for their alleged roles planning and aiding the September 11 2001 terrorist attacks.

This week is the seventh round of pre-trial hearings since their May 2012 arraignment to resolve preliminary legal issues before a trial that is still possibly years away. They could receive the death penalty if convicted.

Cheryl Bormann, who represents Yemeni defendant Walid bin Attash, said the judge could resolve the issue by removing the prospect of the death penalty.

"You can't gag somebody, and then want to kill them," said Ms Bormann, a Chicago lawyer who cloaks herself in a head-to-toe black garment known as an abaya in court to avoid offending her Muslim client.

Prosecutors insist that restrictions on the handling of classified evidence are meant to protect national security and that there has been no violation of the men's rights under the Convention Against Torture.

Clay Trivett, a civilian prosecutor, said the US had adequate safeguards that allowed anyone in custody to pursue allegations of torture.

"They have means available to them," he said. "They just do not have the means available to them they prefer."

The judge, army colonel James Pohl, questioned whether the men, who were subjected to treatment that included prolonged sleep deprivation and the simulated drowning technique known as waterboarding, would complain to the same government that imprisoned them.

But he also seemed sceptical that the lawyers could disclose classified information to third parties even if he altered or removed the protective order.

He did not issue a ruling and the hearing was resumes today.