Alexander Dyukov, the Gazprom Neft president, said that his company would need to seek approval of Serbia's anti-monopoly agency for the deal to buy NIS, the Russian Interfax news agency reported.
Dyukov said the Serbian agency has 130 days to consider the request, meaning the deal will be completed by May.
While the deal, which extends Russia's influence in eastern Europe, was welcomed in the Kremlin, it has divided policymakers in Serbia, even triggering the resignation of Mladjan Dinkic, the economy minister.
Critics say Russia has given no firm guarantees that it will build the natural gas pipeline, called South Stream, by 2015.
The $13bn pipeline under the Black Sea would carry Russian natural gas to Bulgaria and Serbia before branching out to points in western Europe.
Dinkic and other pro-Western politicians fear that Russia's takeover of Serbia's energy sector will dramatically increase Moscow's political influence in the small country.
They also say the $550 million for NIS is too low and that the deal is a payback for Russia's support of Serbia's bid to retain its claim to the breakaway province of Kosovo, which declared independence in February.
Igor Sechin, the Russian deputy prime minister rejected that, saying the deal involved a "market price".