US attempts to move forward in the trial of five Guantanamo prisoners accused of plotting the 11 September attacks have been thwarted for months by scheduling conflicts, religious observances, an Internet outage and a tropical storm.

But the judge has rejected a request to further delay the military tribunal proceedings because of a rat and mould infestation in the offices assigned to defence lawyers at the Guantanamo Bay US Naval Base in Cuba.

A week of pre-trial hearings is set to begin at the base on today for Khalid Sheikh Mohammed, the alleged mastermind of the hijacked plane attacks that killed 2,976 people in the United States in 2001, and four alleged al-Qaeda conspirators accused of providing money and other support for the hijackers.

After an aborted attempt to try them at Guantanamo in 2008, the five were arraigned on revised charges during a chaotic 13-hour hearing in May that was marked by defiant outbursts from the defendants.

There have been no hearings in the case since then.

A hearing tentatively set for June was delayed because one of Mohammed's defence lawyers, a civilian death penalty expert, had to attend the execution of a client in an unrelated case.

A July session was postponed to allow the defendants to observe the holy month of Ramadan, when Muslims fast during daylight hours.

Hearings set for August were delayed when an Internet outage left the lawyers unable to access their electronic legal documents, and then cancelled altogether as Tropical Storm Isaac approached.

It doused the base but caused no damage.

In late September, the work space used by the defence lawyers was shut down because it was infested with mould, rat droppings and at least one decaying rat carcass. Defence lawyers said the 1940s-era building was making them sick.

"My eyes burn. I lose breath," said Cheryl Bormann, a civilian lawyer for defendant Walid Bin Attash.

"All of us suffer some sort of symptoms."

They asked for a delay, saying that without their work area they did not have resources to prepare for the hearings.

The judge, Army Colonel James Pohl, ordered a cleanup but ruled on 5 October that there would be no further postponement.

The building has since been certified as safe by the base hospital's Industrial Hygiene Department, though some of the lawyers said it was still toxic and are working from a cramped high-security trailer inside the court complex rather than return to that building.

Many buildings at the humid and tropical base are kept uncomfortably cold to keep the mold and vermin at bay.

The chief prosecutor, Army Brigadier General Mark Martins, acknowledged to journalists on Sunday evening that the tribunals

were taking place in a "relatively austere setting," but said, "Justice is not determined by the plushness of the surroundings."

Many of the issues the court will address during this week's hearing pertain to secrecy issues.

Defence lawyers want the judge to abolish a "presumptive classification" process that treats as a top national secret any discussion of what happened to the defendants during interrogations in secret CIA prisons before being sent to Guantanamo in 2006.

The judge will also hear news organizations' request to limit closing of the courtroom for secret sessions, and be asked to decide whether the US Constitution governs the tribunals being held at the US base in southeast Cuba.

In addition to Mohammed and bin Attash, defendants Ramzi Binalshibh, Ali Abdul Aziz Ali and Mustafa al Hawsawi, are charged with conspiring with al-Qaeda, attacking civilians and civilian targets, murder in violation of the laws of war, destruction of property, hijacking and terrorism.

All five could face the death penalty if convicted.