Russia’s government revenue from oil and gas sales in October is expected to fall about 21 percent to 950 billion rubles ($11.7 billion) from a year earlier due to lower prices and a stronger ruble, according to Reuters estimates, hitting Moscow’s biggest source of revenue.

The projected reduction will pose more challenges for the Russian government, which is facing a growing public budget deficit and has been forced to raise taxes to finance rising military and security spending.

Oil and gas revenues account for up to a quarter of Russia’s budget and are the most important source of money for Moscow’s military campaign in Ukraine, now in its fourth year.

On a month-on-month basis, October earnings are expected to rise 63 percent as companies pay taxes on profits, according to Reuters calculations based on data on oil and gas production, refining and supplies in domestic and international markets.

Revenue in the first ten months of the year is expected to fall 20.5 percent to 7.56 trillion rubles, Reuters calculations show.

The finance ministry plans to release estimates of oil and gas sales revenue on November 5.

The ministry initially forecast revenue from oil and gas sales to reach 10.9 trillion rubles, or 0.5 percent of GDP, but cut that to 8.32 trillion rubles.