300 Rise of an Empire

Is it a prequel? Or is it a sequel? Different sources would have you believe either way.
IMDB says: “After its victory over Leonidas’ 300, the Persian Army under the command of Xerxes marches south towards the major Greek city-states.” – which would suggest it’s a direct sequel. Eva Green, Lena Headey, and Rodrigo Santoro return while the poster hints at the possible naval Battle of Artemisium.