NEW YORK » The U.S. military has detained more than 200 Afghan teenagers who were captured in the war for about a year at a time at a military prison next to Bagram Airfield in Afghanistan, the United States has told the United Nations.

The U.S. State Department characterized the detainees held since 2008 as “enemy combatants” in a report sent every four years to the United Nations in Geneva updating U.S. compliance with the U.N. Convention on the Rights of the Child.

The U.S. military had held them “to prevent a combatant from returning to the battlefield,” the report said.

A few are still confined at the Detention Facility in Parwan, which will be turned over to the Afghan government, it said. “Many of them have been released or transferred to the Afghan government,” said the report, distributed this week.

Most of the juvenile Afghan detainees were about 16 years old, but their age was not usually determined until after capture, the U.S. report said.

If the average age is 16, “This means it is highly likely that some children were as young as 14 or 13 years old when they were detained by U.S. forces,” Jamil Dakwar, director of the American Civil Liberties Union’s human rights program, said Friday.

“I’ve represented children as young as 11 or 12 who have been at Bagram,” said Tina M. Foster, executive director of the International Justice Network, which represents adult and juvenile Bagram detainees.

“I question the number of 200, because there are thousands of detainees at Parwan,” Foster said Friday. “There are other children whose parents have said these children are under 18 at the time of their capture, and the U.S. doesn’t allow the detainees or their families to contest their age.”

Dakwar also criticized the length of detention, a year on average, according to the U.S. report.

“This is an extraordinarily unacceptably long period of time that exposes children in detention to greater risk of physical and mental abuse, especially if they are denied access to the protections guaranteed to them under international law,” Dakwar said.

The U.S. State Department was called for comment on the criticism, and a representative said they were seeking an officer to reply.

The previous American report four years ago provided a snapshot of the focus of the U.S. military’s effort in the endgame of the Bush presidency after years of warfare and anti-terrorism campaigns. In 2008, the U.S. said it held about 500 juveniles in Iraqi detention centers and then had only about 10 at the Bagram Airfield in Afghanistan. A total of some 2,500 youths had been detained, almost all in Iraq, from 2002 through 2008 under the Bush administration.

Barack Obama campaigned for the presidency in 2008 in part on winding down active U.S. involvement in the Iraq War, and shifting the military focus to Afghanistan. The latest figures on under-18 detainees reflect the redeployment of U.S. efforts to Afghanistan.

Because the teen detainees were not charged with any crime, “a detainee would generally not be provided legal assistance.” They were allowed to attend open hearings and defend themselves, and a personal advocate was assigned to each detainee, the report said.

“These are basically sham proceedings,” Foster said. “The personal representatives don’t do anything different for the child detainees than they do for the adults, which is nothing.”

The report added that “the purpose of detention is not punitive but preventative: to prevent a combatant from returning to the battlefield.”

It cited a 2004 U.S. Supreme Court case, Hamdi vs. Rumsfeld, as establishing that “the law of armed conflict permits the United States to detain belligerents until the end of hostilities without charging such individuals with crimes, because they are not being held as criminals facing future criminal trial.”

The U.S. military is fighting irregular forces — al-Qaida, the Taliban, and an array of similar shadowy insurgent or terrorist groups. So it is not clear when “hostilities” would ever formally end, since there is no declaration of war and no enemy government to defeat. Only the United States can decide when it deems a conflict to be over, in those circumstances.

Foster said that the teens seized are not in uniform or even typically taken in combat.

“We’re not talking about battlefield captures, we’re talking about people who are living at home, and four or five brothers might be taken together. It might take them a year or more to figure out that one of them was younger than 18, to determine the identities of these kids,” she said.

In January, the State Department will send a delegation to Geneva to present the report to the U.N.’s Committee on the Rights of the Child, and to answer any further questions the U.N. committee members may have.