WASHINGTON (Reuters) - The United States sees ongoing Russian activity in eastern Ukraine as a prelude to a much larger offensive in the country, a senior U.S. defense official said on Tuesday.

       The official, who spoke on condition of anonymity, said Russia was still adding to its military capabilities to prepare for the new offensive, introducing an additional two battalion tactical groups (BTG) in the past 24 hours to bring to the total number of Russian BTGs in the country to 78.

       (Reporting by Phil Stewart and Idrees Ali; Editing by Chizu Nomiyama)