The man overseeing prisoners' appeals at Guantanamo Bay has rejected claims that the process is flawed.
England does not expect many inmates to be freed
Referring to criticism that inmates have not been allowed lawyers, US Navy Secretary Gordon England insisted the system was "professional" and "fair".
Journalists are being allowed in on Thursday to observe the tribunals.
Five of the eight prisoners reviewed so far have refused to take part in the process, set up to determine whether they are being held legally.
The tribunals, which have been running since Friday, were instigated after the US Supreme Court ruled that the prisoners could challenge their detentions.
'No declared war'
The BBC's correspondent Nick Childs says the Pentagon's decision to go ahead with the hearings comes after heavy pressure on the US Defence Department.
But with a number of detainees refusing to appear it could make it problematic for the military to argue that the tribunals are a valid process, he says.
Mr England told BBC Radio 4's Today programme that the system was derived from the Geneva Convention.
Journalists are being allowed to view proceedings
"We are allowing the detainees to appear before the hearing and present their case, and we have a person to work with them," he said.
He said the person did not need to be a lawyer, as "this is not a legal proceeding, it's an administrative proceeding".
He said detainees were being treated as enemy combatants, not prisoners of war, because "there's no declared war between countries".
If the tribunals, presided over by three military officers, find that a prisoner does not qualify as an enemy combatant, he may have to be freed.
Mr England conceded this, but said he did not expect "a very large number" to be released.
So far only three of the eight prisoners reviewed have agreed to co-operate in the process. The tribunals' findings in the cases are not yet known.
The defence department says the detainees who refused to appear when called were a Saudi, a Moroccan and three Yemenis, all of whom were trained by al-Qaeda or associated with the group in Afghanistan.
Their names have not been released.
A military spokeswoman said the men had "historically not co-operated".
The first detainee who was reviewed, a 24-year-old Algerian, reportedly said he would "kill Americans" if released.
US authorities say all of the detainees will eventually go through the process, even if they choose not to appear before the tribunals in person.