KYIV, Oct 8 (Reuters) - A Ukrainian presidential advisor posted a message on Twitter after conflicting reports of an explosion or fire on Saturday that damaged the bridge connecting the Russian mainland to the occupied Ukrainian peninsula of Crimea, calling it "the beginning" but not directly claiming Ukrainian responsibility."Everything illegal must be destroyed, everything stolen must be returned to Ukraine, everything occupied by Russia must be expelled," Mykhailo Podolyak wrote.Register now for FREE unlimited access to Reuters.comOur Standards: The Thomson Reuters Trust Principles."