Undersecretary of Defense Peter Levine told reporters at a DoD press conference on Tuesday that there is no good intelligence to measure how many ISIS fighters have been killed over the course of the American war against them. A reporter asked him about a report from a "senior Defense Department official" that claimed the military had killed 50,000 ISIS fighters in Iraq and Syria over the past two years, and he declined to back up those claims. "First of all, again, a senior Defense Department official provided you with that number, which was their prerogative to do. It's not a number that we talk about publicly because it's not a good metric of success, and it’s not an easy number to determine for all the reasons you would think about," Levine said. "This is a warzone. This is a conflict. Much of our efforts are being conducted from the air. We don't have the ability to assess everything on the ground in the way we might like." "There have been a variety of estimates," of how many ISIS fighters have been killed, he said. "There isn't a good number for us to share with you, because we don't have great confidence in that number."