YANGON (Reuters) – A top United Nations envoy on Saturday voiced grave concern over alleged abuses by Myanmar security forces after sectarian violence in Rakhine State and urged a full and credible state investigation. Tomas Ojea Quintana, the U.N. special human rights rapporteur, called on the government to find out the truth about violence in June between ethnic Rakhine Buddhists and stateless Muslim Rohingyas and address reports of extrajudicial killings and torture by its police and soldiers.