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.

KYIV (Reuters) - Ukraine has lost over 40% of the territory in Russia's Kursk region that it rapidly seized 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.

Russian President Vladimir Putin says Moscow has tested a new intermediate-range missile in a strike on Ukraine and that it could use the weapon against countries that have allowed Kyiv to use their missiles to strike Russia.