The United States is holding about 10,500 prisoners in Iraq (search), more than double the number held in October, the military says.

About 100 of those prisoners are under age 18, said Army Lt. Col. Guy Rudisill, a spokesman for detention operations in Iraq.

Five months ago, the military said it was holding about 4,300 prisoners in Iraq. The growth in the prison population has come amid a lingering insurgency in Iraq and despite the formal transfer of power to an interim Iraqi government last June.

The number of U.S.-held prisoners in Iraq declined last summer after international outrage over abuses at the Abu Ghraib (search) prison near Baghdad. Revelations of abuse have continued since then; on Friday, the Army released documents detailing a half-dozen prison abuse investigations in Afghanistan and Iraq.

The reports said soldiers had stripped prisoners naked and released them to walk home, beaten detainees with their fists and feet, broke a prisoner's jaw and forced detainees to exercise to the point of exhaustion. One investigator concluded prisoners at a temporary jail near Mosul (search) were systematically mistreated and possibly tortured in December 2003.

Spokesmen for U.S. Combined Forces Command-Afghanistan did not respond Monday and Tuesday to e-mailed questions about the number of prisoners there. The military estimated in January that it was holding about 500 people in Afghanistan.

A human rights group was issuing a report Wednesday saying the rising number of detainees increases the risk that the prisoners will be mistreated. The report from New York-based Human Rights First says secrecy about the prisoners is also increasing, citing the refusal of military officials to discuss the number of prisoners in Afghanistan since January.

"We're seriously concerned about overburdening of what the Pentagon has called transient facilities, the field prisons," Human Rights First lawyer Deborah Pearlstein said Tuesday. "These are places where conditions are terrible, where the worst abuses occurred from 2002 to 2004, and ICRC [International Committee of the Red Cross] access is limited to nonexistent."

About 1,200 of the prisoners in Iraq are being held at temporary facilities at forward bases, Rudisill said. He said the other 9,300 are held in three permanent prisons: the Abu Ghraib jail near Baghdad, Camp Cropper at the Baghdad International Airport and Camp Bucca in southern Iraq.

In December, U.S. officials said they were holding about 65 prisoners age 16 and under. The 100 or so under-18 prisoners in custody now are being held separately from adult prisoners, Rudisill said.

In an interview transcript made public earlier this month, the former head of prison operations in Iraq described meeting an imprisoned boy who said he was 11 years old but looked more like an 8-year-old. Brig. Gen. Janis Karpinski said the boy was crying for his mother, but did not say what happened to him.