Usually the tactic is to surround the city and cut it off from supplies. Starve it. I fear the Rus leader will nuke it. Since NATO is not going in there with their troops, what does he have to lose? The capital is not what he is after, as others have pointed out, it is the resources he wants. He can claim he saved Europe from the fascists, Nazis, and other imaginary evils.
I am not sure how to deescalate this situation using conventional military strategy and tactics. Maybe bribe his regime to oust him? Average Rus salary is $26,500 per annum. Draftees get paid $30 month. $3000 a month is a great wage in Rus.