MOSCOW—Russia's OAO Gazprom , GAZP.RS -0.50%the world's largest producer of natural gas, said Friday its earnings increased by 35% in 2011 to nearly $45 billion, reflecting higher energy prices and growing exports.

Analysts, however, were unsure of the company's future prospects, as the government plans to raise taxes for the gas industry to help fund a massive wave of populist spending promised by President-elect Vladimir Putin.

Russia plans to raise natural-gas prices by 15% next year—less than hoped for by Gazprom, which has for years lobbied the government to quicken the pace of price increases for domestic customers. An estimated 80% of the price rise, however, will be offset by a planned increase in the country's Mineral Extraction Tax.

"This news [on taxes] is not to be welcomes by investors, especially given the overall negative mood in the market," VTB Capital said.

Others, however, were more optimistic, saying the additional burden would force Gazprom to develop more effective strategies regarding future spending.

"Tax increases should force Gazprom to reduce its investments, the bulk of which, we calculate, are wasteful, and what is left for the minority shareholders would be relatively unchanged," Troika Dialog said.

Gazprom's net income attributable to shareholders rose to 1.31 trillion rubles ($44.6 billion) from 968.6 billion rubles in 2010.

Sales rose to 4.64 trillion rubles from 3.6 trillion rubles in 2011.

The company widened its share of the European gas market to 27% last year, from 24% in 2010, as sales increased to 150 billion cubic meters, from 139 billion cubic meters.