LAGOS, Nigeria — The United Nations has negotiated the release this year of 876 children detained at a Nigerian army barracks holding suspected collaborators of the Boko Haram Islamic extremist group, the U.N. Children's Fund announced Friday.The agency fears hundreds more children are still being held at the barracks in the northeastern city of Maiduguri, the UNICEF spokeswoman for Nigeria, Doune Porter, told a news agency.This is the first time the U.N. has reported negotiating the releases, though Nigeria's army routinely reports how many minors are among the hundreds of detainees it frees after interrogations it says establish they have no links to Boko Haram.Some of the 876 children released since December had been living in areas held by Boko Haram and were detained when those areas were liberated, according to Manuel Fontaine, UNICEF's director for West and Central Africa.Porter said many of the freed children were under 5 years old, some still being breast-fed, and were detained because their parents were suspects. Nigeria's military and police routinely lock up children along with parents suspected of a crime.In the biggest single release negotiated by UNICEF, 560 people were freed in September, including 430 children and some of their mothers, Porter said.Those detained have been held in Maiduguri, the city that is the birthplace of Boko Haram and the home of the Nigerian army's Giwa Barracks. All of the detainees at the barracks are held because of suspected support for Boko Haram.The Associated Press has documented the deaths of thousands of detainees in unsanitary, overcrowded and inhumane conditions at Giwa Barracks in recent years. Amnesty International has said 8,000 detainees died there between 2011 and 2015. This year, Amnesty called for the detention center's closure, saying babies and children are among the many detainees dying from disease, hunger, dehydration and untreated gunshot wounds.Ministry of Defense spokesman Brig Gen. Rabe Abubakar has called the charges by the London-based human rights group "a distraction," insisting that "our duty is to protect lives, and that is what we have been doing."Once freed from detention, the often malnourished and traumatized children face other challenges, Porter said. "All of the people who have been held in territories that were occupied by Boko Haram face a lot of distrust and fear from their communities, so they have been widely regarded with suspicion."Boko Haram's use of child suicide bombers, often females, has contributed to the fear.The extremist group's seven-year uprising has killed more than 20,000 people, spread across Nigeria's borders and forced 2.6 million from their homes.