Tina Foster, the executive director of the IJN, said: "It's shocking to me that the US government has not figured out a way to keep children out of adult prisons.
"It's outrageous, and it is not making us any safer. I can say that about Afghanistan from personal experience."
Her group brought lawsuits on behalf of the Guantanamo detainees in 2006, and has taken on the cases of adult detainees in Bagram.
In the periodic report to the United Nations on US compliance with the Convention on the Rights of the Child, the US confirmed that "as of April 2008, the United States held about 500 juveniles in Iraq".
"The juveniles that the US has detained have been captured engaging in anti-coalition activity, such as planting improvised explosive devices (IEDs), operating as lookouts for insurgents, or actively engaged in fighting against US and coalition forces," the US report said.
The majority are believed to be 16- or 17-years old. In the US a 17-year old can enlist in the army, with parental consent.
The report said that of the total of 2,500 juveniles jailed since 2002, all but 100 had been picked up in Iraq.
"It's shocking to me that the US government has not figured out a way to keep children out of adult prisons"
Tina Foster, executive director, International Justice Network
The vast majority of the remainder were detained in Afghanistan.
A total of eight juveniles have been held at the Guantanamo Bay prison, but all were released between 2004 and 2006.
"It remains uncertain the exact age of these individuals, as most of them did not know their date of birth," the report says.
But US military doctors who evaluated them believed that three were under the age of 16.
In Afghanistan, "... as of April 2008, there are approximately 10 juveniles being held at the Bagram Theatre Internment Facility as unlawful enemy combatants," the report said.
A US military spokesman in Bagram, Marine 1st Lieutenant Richard Ulsh, told the Associated Press: "At any time there are up to 625 detainees being held at the Bagram Theatre Internment Facility.
"There are no detainees being held under the age of 16 and, without getting into specifics due to the frequent fluctuation in the number of detainees being held, we can tell you that there are currently less than 10 detainees being held under the age of 18."