The United Nations has said, an estimated 3,900 children were killed by the Nigeria-based Islamic extremist group Boko Haram during the last four years. In a report it said, another 7,300 children were wounded.<br/><br/>The report said, at least 7,000 girls and women have suffered sexual violence since 2009. UN Secretary-General Antonio Guterres has urged the Nigerian government to try to minimize the number of children victimized during its military campaign against the extremist group.