NEW : Head of Amnesty International refugee agency 'deeply distressed ' U.N. says peacekeepers have failed to prevent ethnic cleansing . CNN 's Nana Karikari-apau contributed to this report . Violence has led to a `` Christian exodus of historic proportions , '' says the rights group . Muslim and Muslim militia groups are warring in Seleka .
NEW : Head of Amnesty International refugee agency 'deeply distressed ' U.N. says peacekeepers have failed to prevent ethnic cleansing . Violence has led to a `` Christian exodus of historic proportions , '' says the rights group . Muslim and Muslim militia groups are warring in Seleka .