(Reuters) - Russia decided to blockade the besieged Azovstal steel plant in the city of Mariupol because it cannot take it by force, Oleksiy Arestovych, Ukraine's presidential adviser said on Thursday.

       "They physically cannot take Azovstal, they have understood this, they have taken huge losses there. Our defenders continue to hold it," Arestovych said at a briefing.

       It "can also be explained by the fact that they have moved part of their forces (from Mariupol) to the North in order to reinforce the troops attempting to fulfill their main objective... advancing to the administrative boundaries of Donetsk and Luhansk regions."

       (Reporting by Max Hunder; Writing by Aleksandar Vasovic; Editing by Toby Chopra)