More than 500 prisoners are being held at Guantanamo Bay

The Senate Judiciary Committee heard that US treatment of those held at the prison was harming its image abroad.

It was also told by a government official that the US had the legal right to keep terror suspects in custody forever.

The issue of terror suspects has become deeply divisive in the US.

The BBC's Adam Brookes reports that the atmosphere at the Senate hearing was uncomfortable.

Like much of the US, he says, the committee seems to be increasingly uneasy at the treatment of prisoners caught during the "war on terror".

Several senators and former President Jimmy Carter have called for the military prison at Guantanamo Bay to be closed down.

More than 500 men are being held at the facility. Many have been there for more than three years without trial.

'Held in perpetuity'

Democratic Senator Patrick Leahy told the committee that Guantanamo Bay "is causing immeasurable damage to our reputation as a defender of democracy and a beacon of human rights around the world".

But William Barr, a former US Attorney General, defended the Bush administration's decision to exempt terror suspects from the Geneva Convention.

"The Geneva Convention applies to signatory powers," he said. "Al-Qaeda hasn't signed it. They're not covered by the Geneva Convention."

Democratic Senator Joseph Biden asked the Deputy Associate Attorney General, J Michael Wiggins, if suspects could be held forever at Guantanamo Bay because "there is no definition as to when the conflict ends".

Mr Wiggins replied: "It's our position that, legally, they can be held in perpetuity."

Our correspondent says this suggests the Bush administration believes that people who pose a threat to the US may never be released from custody.