People walk along a street past a board showing currency exchange rates in Moscow on December 3 Reuters

Russia's central bank has offered to help exporters refinance foreign debts in 2015, as it seeks to steady the ruble.

The bank said it would lend dollars and euros to big companies that would put up their foreign borrowing as collateral.

In effect, the move would see Russia's central bank taking on the credit risk for private companies, the latest in a series of state intervention in the economy.

Meanwhile, credit ratings agency Standard and Poor's placed Russia's sovereign credit outlook on "credit watch negative", raising the possibility it could be downgraded to junk in January.

S&P, Moody's and Fitch have now all placed Russia just one notch above junk status.

Russia's state debts may be minimal but its private companies and banks are due to repay around $100bn of its debts in 2015, according to Reuters.

"We note that S&P's rating for Russia is currently at the low boundary of the investment-grade category and would lose such a status if downgraded," analysts at Sberbank Investment Research wrote in a note.

"Things would become more complicated if two of three ratings are moved to speculative."