"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.

All of the Ukrainian troops need treatment, Ukraine's President Volodymyr Zelenskiy said in a message on the Telegram messaging app.