WASHINGTON (Reuters) - The United States military has started training a small number of Ukrainian troops on using howitzer artillery, a senior U.S. defense official said on Wednesday, adding the training was being conducted outside of Ukraine and would take about a week.

       "It's a smallish number of Ukrainians, a little bit more than 50," the official said, speaking on condition of anonymity.

       Last week, U.S. President Joe Biden announced an additional $800 million in military assistance to Ukraine, expanding the aid to include heavy artillery ahead of a wider Russian assault expected in eastern Ukraine.

       (Reporting by Phil Stewart and Susan Heavey)