"Allies of the US clearly believe that Guantanamo is doing the US's reputation a great deal of damage.
"It actually makes it more difficult for them to pursue an effective counter-terrorism strategy."
The concrete-and-steel facility was originally designed as a medium-security prison.
Detainees confined in individual cells will now look out through long, narrow windows on areas with metal tables and stools that were originally intended to be shared living spaces, but which will now be off-limits.
Shower doors have been redesigned so prisoners' hands and feet can be shackled by guards before they leave the stalls.
"Anti-jump" fencing has also been installed along catwalks.
An open-air recreation area has been divided into smaller spaces, which will hold only one detainee at a time.
Guantanamo officials say the 178-cell facility aims to isolate prisoners and reduce their ability to communicate with each other.
Kris Winter, a US navy commander, said the the modifications will help to protect guards after a clash between detainees and guards in May.
Robert Durand, director of public affairs, Joint Task Force Guantanamo, said: "The new, climate-controlled camp is designed to improve life for both detainees and the guard force."
Human rights organisations have said that "prolonged periods of isolation constitute cruel and inhumane treatment".
Since prison facilities at Guantanamo Bay were first established in 2001, the US has faced calls for the camp's closure on grounds of human rights infringements.
Porteous said: "Clearly some of the detainees have been mistreated. There have been serious allegations, but there has been no serious
|Since the facilities were first established|
the US has faced calls for its closure
US officials say the new facility is necessary to carry out their mission of holding men the US calls "enemy combatants", but the legal status of Guantanamo's inmates is in doubt.
Porteous said: "The term 'enemy combatant' is dubious at best. It's clearly a device being used to keep these people in a legal black hole."
Human rights groups have pointed to repeated hunger strikes and suicide attempts by inmates as acts of desperation.
Harry Harris, the commander of Guantanamo's Camp 1, described suicides in his camp on June 10 as "an act of asymmetric warfare against us ... not an act of desperation".
"Designed to improve life"
About 100 men who have been cleared for release and are awaiting transfer to another country are among more than 400 currently being held at Guantanamo.
Many are likely to face mistreatment or torture at the hands of their own governments if they return home.
Under international law, the US cannot return them to a country where they will be tortured.
Human Rights Watch has suggested that a third country should step in to offer asylum.
Source: Al Jazeera and agencies