Thousands of Russian troops have died in the less than two weeks since they invaded Ukraine, according to U.S. intelligence agencies. However, their limited presence and the fog of war make accurate estimates difficult.

Lt. Gen. Scott David Berrier, director of the Defense Intelligence Agency, told the House Intelligence Committee on Tuesday that his agency has seen estimates that between 2,000 and 4,000 Russian troops have died. He also noted they have “low confidence” in the assessment, gathered from “some intelligence sources, but also open source” data.