Oil futures remained under pressure in early trading on Tuesday, following a slide that has seen prices fall by more than a quarter since the beginning of the year, as the full return of Iran to oil markets adds to an already huge supply overhang. Oil prices had briefly stabilized in the previous session, but only after hitting the lowest since 2003 as western sanctions against Iran were lifted, allowing the country with the world’s fourth-largest oil and gas reserves to return in full to the market.