Relatives of those killed on 9/11 will gather today in a courtroom on Guantanamo Bay to to see the mastermind of the Twin Tower attacks appear before a US military tribunal.

Khalid Sheikh Mohammed, and four co-defendants who face the death penalty on charges relating to the attacks, will appear in a pretrial hearing at the military base even as the base itself faces closure.

Five relatives of 9/11 victims will look on as Mohammed and his co-defendants question the judge over-seeing the case over his ability to be impartial.

It will be the first time that relatives of the 2001 attacks have been allowed into a military hearings at Guantanamo.

President-elect Barack Obama opposes the Guantanamo military trials and has pledged to close the detention centre holding some 250 men soon after taking office next month.

With no trial date set, it is unlikely one will begin before Mr Obama takes office on January 20 and it is possible that a trial will never materialise. But the US military is pressing ahead with the case until it receives orders to the contrary.

"We serve the sitting president and will continue to do so until President-elect Obama takes office," said Navy Commander Jeffrey Gordon, a Pentagon spokesman.

The victims' family members, who were chosen in a Pentagon lottery system, will sit in a viewing gallery at the back of the cavernous courtroom, separated from proceedings by an acrylic glass wall and an audio time-delay.

The time-delay allows the court's security officer to cut the audio feed if information considered classified is mentioned.

Maureen Santora, whose son Christopher, a fire fighter was killed at the World Trade Center, said she wanted to to lock eyes with those accused of killing her son and 2,972 others in the bloodiest terrorist attacks ever on U.S. soil.

Relatives of about 30 more victims, mainly firefighters, have given her Santora memorial cards that she planned to bring into court "to know their spirit is with us."

Mohammed came up with the idea of hijacking arilines over the US and flying them into iconic buildings in 1999.

It took him months to persuade Osama bin Laden that the plan could work but after the attacks the US authorities publicly identified him as the driving force behind the attacks and placed a $25 million price on his head.

Mohammed, who also plotted to attack Heathrow and blow up Big Ben, was arrested in 2003 when Pakistani police, acting on a tip-off, raided a house in Rawalpindi where he was sleeping in an upstairs room.

After his transfer from one of the CIA's "ghost prisons" to Guantanamo Bay last year the American authrorities described him as "one of history's most infamous terrorists".

Mohammed and the other defendants will appear before Colonel Stephen Henley, who was assigned to the case after the previous judge resigned for undisclosed reasons in November.

The defendants, who are representing themselves, are expected to question Colonel Henley about whether any conflicts would prevent him from impartially overseeing the death-penalty case.

The military commissions have netted three convictions, but have been widely criticized for allowing statements obtained through harsh interrogations and hearsay to be admitted as evidence.

Today's session is expected to have moments of hight drama. Mohammed has previously said he wants to be executed and achieve martyrdom, but has still mounted a vigorous defence, addressing the court and asking for materials to prepare his case.

If Mr Obama decides to close Guantanamo, authorities may have to release accused foreign terrorists to their home countries.

Others might be prosecuted in US criminal courts, or in a special hybrid court that combines US civilian criminal law and military tribunals -- an idea which many have condemned as inworkable.

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