Three children were executed by the M23 rebel group in the Democratic Republic of Congo (DRC), the UN has said, as troops loyal to the government retreat from key cities. The Rwandan-backed rebels have been leading an insurgency in the central African country since last month. The children were executed in the eastern city of Bukavu, the UN Human Rights Office (UNHRO) said, which rebels reportedly seized on Sunday. It has previously accused the M23 group of recruiting children. The UN said in a statement Tuesday that his office had "confirmed cases of summary execution of children by M23 after…