MOSCOW, May 20 (Reuters) - Russia's state oil and gas revenues, which account for around a fifth of total budget income, are set to ​rise by 39% year on year in May to ​700 billion roubles ($9.8 billion), thanks to a global oil price ⁠rally fuelled by the war in Iran, Reuters calculations showed on ​Wednesday. However, revenues are expected to decline from April by around 17% ​due to cyclical payments of profit-based tax. Russia's budget will also lose out from increasing subsidies to the refineries in the form of the reverse excise ​tax and damper payments.