* U.S. says 200 Afghan teens held at Bagram jail since 2008
* Most released or handed over to Afghan authorities
* Activists say juveniles not given special treatment
GENEVA, Dec 8 The United States held more than 200 Afghan teenagers captured in the conflict with al-Qaeda and the Taliban for an average of a year before releasing them or handing them over to local authorities, a U.S. report to the United Nations this week said.
U.S. authorities said the average age of those detained was 16. However, in nearly all cases their age was not determined until after the juveniles were already in custody at a U.S. military prison at Bagram airbase and medically examined.
"Over the last several years the United States has captured more than 200 individuals under the age of 18 and held them at the Detention Facility in Parwan (DFIP)," said the report to the U.N. Committee on the Rights of the Child, referring to the prison facility next to the Bagram base.
"Few of these juveniles remain in detention at the DFIP; many of them have been released or transferred to the Afghan government," it said, without giving figures.
"Generally, for juveniles, the average length of stay at the DFIP has been approximately one year."
The report dated Dec. 3 and posted on the U.N. committee's website, responded to questions from U.N. independent experts regarding the detention of captives under 18 since 2008, charges laid, length of custody and legal assistance made available.
Washington handed over the high-security Bagram prison and around 3,000 suspected Taliban fighters to Afghan control over a six-month period ending on Sept. 10, but did not transfer 600 or so detainees captured after the memorandum was signed.
Afghan President Hamid Karzai last month demanded the release of dozens of prisoners from U.S. custody and said all inmates held in breach of an agreement must be transferred immediately to Afghan authorities.
The prison deal helped lay the ground for a separate strategic pact framing U.S. development aid and support to Afghanistan for a decade after the withdrawal of most foreign combat troops in 2014.
The U.S. Supreme Court ruled in 2004 that detaining enemy combatants for the duration of the armed conflict "is a fundamental incident of waging war," the U.S. report said, adding this was consistent with the Geneva Conventions.
"The purpose of detention is not punitive but preventative: to prevent a combatant from returning to the battlefield."
In line with the law of armed conflict, the great majority of combatants detained by the United States in Afghanistan, including juveniles, will not face criminal charges, it added.
Administrative hearings have been held periodically to assess whether a juvenile captive should continue to be held.
"Every effort is made to provide them a secure environment, and to provide the special physical and psychological care that they may need," it said.
Human Rights Watch and Human Rights First, in a separate report to the U.N. committee, said in late March 2012, about 250 children under age 18 were held at Bagram.
"Children aged 16 and 17 were co-mingled with adults and not afforded any special education or rehabilitation programme nor any review procedures that took into account their juvenile status," the U.S.-based activist groups said.
The International Committee of the Red Cross (ICRC) - whose confidential findings are given only to the detaining authorities - visits detainees in U.S. custody and facilitates phone calls and mail with their families, the U.S. report said.
The U.N. committee will examine the U.S. report and further question a U.S. delegation about its policies regarding children at home and abroad at its Jan. 14-Feb. 1 meeting in Geneva. (Reporting by Stephanie Nebehay; Editing by Sophie Hares)