Ratings agency Standard and Poor's has indicated it may upgrade its rating on Rosneft following the Russian oil company's agreement to buy a joint venture between BP and a group of Russian billionaires.

Rosneft on Monday agreed to buy TNK-BP, Russia's third-largest oil producer. It is buying 50 percent from British oil company BP for about $17 billion in cash and a 12.84 percent of its own shares. Rosneft is buying the other half for $28 billion from a group of Russian shareholders.

S&P said in a statement Wednesday night that it had revised its status on the company from negative to positive, meaning that an upgrade is likely. The agency said it would take action if it sees a "likelihood of extraordinary government support" offsetting a higher debt burden.