"Even if you're on the ground, it's very difficult for you to count casualties," said Marina Miron, a postdoctoral researcher at King's College London.

The Ukrainian military says Russia’s military losses have topped 1 million, noting the huge price Moscow has paid for its 3-year-old invasion.

With high casualty figures and the slow pace of Russia’s territorial gains, President Vladimir V. Putin could face years more of a grinding war of attrition in Ukraine.