Russia has made slow but steady gains in recent weeks against Ukraine but at a steep cost — the bloodiest month in the nearly three-year war, U.S. officials said Wednesday.

Russia has sustained more than 600,000 casualties during the war in Ukraine, a sign of losses accelerating out of proportion with its gains, Pentagon officials said. Since the summer, Moscow has continued to take territory in Ukraine's eastern region of Donetsk,

Russian forces have made "significant progress" in parts of the Donetsk region, according to an open-source intelligence outlet.