New York - The UN chief has fired the head of the peacekeeping mission in the Central African Republic over the force's handling of dozens of misconduct allegations, including rape and killing.Secretary-General Ban Ki-moon said on Wednesday he has accepted the resignation of Babacar Gaye of Senegal."Enough is enough," he said.He has called a special session of the UN Security Council for Thursday over the issue of sexual abuse allegations."I cannot put into words how anguished and angered and ashamed I am by recurrent reports over the years of sex abuse and exploitation by UN forces," said Ban, who first heard about the latest allegations Tuesday, a week after the first UN officials were informed.He said he also will hold a special meeting Thursday with the heads of all peacekeeping missions around the world to stress their responsibilities.11 cases of possible sexual abuseBan's spokesperson, Stephane Dujarric, called the firing of such a high-level official "unprecedented". He said the peacekeeping force in Central African Republic has faced 57 allegations of possible misconduct, including 11 cases of possible sexual abuse, since the mission was established in April 2014 to calm deadly violence between Muslims and Christians. Ban appointed Gaye in July.Dujarric said he did not know how many peacekeepers are involved in the 57 allegations.Gaye did not respond to requests for comment. His resignation letter, seen by The Associated Press, says the issue "could be a systemic problem warranting consideration at the highest level of the organisation."Gaye's letter also said he had taken a "very robust stand" against misconduct and that his mission had repatriated "many" peacekeepers for misconduct. The spokesperson for the mission, Hamadoun Toure, told the AP that at least six were sent home last month alone.The firing comes a day after Amnesty International accused U.N. peacekeepers in Central African Republic's capital of indiscriminately killing a 16-year-old boy and his father and raping a 12-year-old girl in separate incidents this month.That follows allegations that UN peacekeepers had sexually abused street children in Bangui and a separate allegation of child sexual abuse against a peacekeeper in the eastern part of the country.Powers of criminal investigationIn an email Wednesday, the legal director of Medecins Sans Frontieres, Francoise Bouchet-Saulnier, said that "since September 2014, MSF has treated a total of four minors who reported sexual abuse by the UN peacekeeping forces in three separate cases" in Central African Republic. Bouchet-Saulnier said the case included the 12-year-old girl.The UN mission in Central African Republic is also being investigated over how it handled child sexual abuse allegations against French troops last year, in which children as young as 9 said they had traded sex for food.In a statement, the US-based advocacy group AIDS-Free World blasted what it called the "monumental failure of leadership and appalling mismanagement" at the UN headquarters.Ban in the past has expressed his interest in starting to "name and shame" the countries whose peacekeepers are accused of misconduct, though calling out countries has its risks. The UN has no standing army and relies on member states to contribute troops and police for its missions."I want member states to know that I cannot do this alone," Ban said on Wednesday. "They have the ultimate responsibility to hold individual uniformed personnel to account."The UN has no powers of criminal investigation or prosecution, leaving it up to peacekeepers' home countries — which the UN officials often don't name publicly.