"Casualty estimates for the conflicts in Iraq and Syria are inconsistent and unreliable because of a range of methodological challenges in conducting and reporting counts. Estimates of the number of people who have died during Syria's civil conflict since March 2011 range from 250,000 to 470,000. In Iraq, the estimated range is between 19,000 and 41,650 deaths since January 2014. This product discusses the difficulties of collecting war-related casualty data in both countries and provides an overview of some of the current estimated figures available through selected organizations. CRS [Congressional Research Service] is unable to independently track casualties in Syria and Iraq, and cannot verify the data presented."
CRS Insight, IN10474
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html