KYIV (Reuters) - Ukraine has lost over 40% of the territory in Russia's Kursk region that it captured in a surprise incursion in August as Russian forces have mounted waves of counter-assaults, a senior Ukrainian military source said.

Tit-for-tat moves this week included the use of American-made ballistic missiles to strike inside Russia, and new nuclear threats from Moscow. Neither appear to have influenced the war on the ground.