With anti-US violence in Iraq continuing to rage, American forces are now holding more than 10,000 people they classify as "security detainees" in their three main jails in Iraq.
This is nearly double the number they held a year ago.
The new constructions will give the United States the capacity to hold up to 16,000 people in Iraq, military spokesman Lieutenant Guy Rudisill said.
US forces will build a new prison at a former military barracks in Sulaimaniya, 330km north of Baghdad, and add room for 2000 more prisoners at Camp Cropper near Baghdad airport, which now holds just 125 detainees including former President Saddam Hussein.
The biggest US-run prison, Camp Bucca near Umm Qasr in the south, will also be expanded to hold an additional 1400 detainees.
Abu Ghraib - Saddam's most notorious prison which also became the focus of a scandal for US troops after pictures emerged showing them sexually abusing detainees there - has just been expanded to house 400 more detainees and will get room for another 400.
US officials said after the scandal that they hoped to close Abu Ghraib. Rudisill said the plan is to move prisoners from there to Cropper after that camp is expanded.
"All security detainees held by the multinational forces are considered a threat to the security and stability of Iraq"
US army spokeswoman
The US-run camps are for "security detainees" held by Iraqi and US-led international forces as suspected "insurgents". Ordinary Iraqi criminals are held in regular Iraqi jails.
Rudisill said the expanding US-run prison camp population in Iraq is a result of "successful military operations against the insurgency and terrorists by coalition forces and the Iraqi special forces".
However, US authorities in Iraq have come under harsh criticism for conditions in their jails and also for the alleged incarceration of innocent people there.
In a recent report, New York-based Human Rights Watch said "harsh and coercive interrogation techniques such as subjecting detainees to painful stress positions and extended sleep deprivation have been routinely used in detention centres throughout Iraq".
The US army told Aljazeera.net that "all security detainees held by the multinational forces are considered a threat to the security and stability of Iraq".
Many Iraqis say innocent people
are languishing in US jails
An army spokeswoman said: "To ensure detainees receive due process, a board of Iraqi and US officials looks at each security detainee's case within the first 90 days of their detention to see if they no longer pose a security threat and are eligible for release. Detainees' cases are then reviewed every 180 days."
She added that the US will release detainees when they are no longer deemed a security threat, have no further intelligence value, and do not merit criminal prosecution.