Bulgarian leaders have complained that Bulgarian oil refiner Lukoil Neftohim hasn't installed monitoring devices that would provide information regarding what kind of gas is released to the market.
A former interior minister, meanwhile, deflected claims that he played a role in alleged fuel smuggling through Lukoil, one of Russia's largest oil companies.
Sofia noted that Lukoil was raising energy prices for the country. Gasoline prices were up 17 percent since January, while the price of diesel fuel had increased 13 percent, the Sofia News Agency reports.
Energy prices on world markets have slowly increased throughout the year, in part because of global economy recovery and from conflict in war-torn Libya, one of Africa's largest oil producers.
Europe is keen to break Russia's grip on the regional energy sector. Sofia said earlier this year it was looking for a better deal with Russian gas monopoly Gazprom, which supplies roughly one-quarter of Europe's natural gas.