(Reuters) - Russia's central bank offered on Wednesday to help top exporters refinance heavy foreign debts next year, expected to be one of the toughest of President Vladimir Putin's 15-year rule for the economy due to Western sanctions and a plunge in oil prices.

The bank said it would help major Russian companies to refinance foreign debts by lending dollars and euros to those who were willing to put up their foreign borrowings as collateral.

The move means that the state will in effect take on credit risk for the companies, whose foreign debt obligations have shot up in ruble terms because of the currency's sharp slide this year.

Even before the move, Standard and Poors put Russia's sovereign credit outlook on "creditwatch negative," meaning it could downgrade it to junk as soon as January due to a "rapid deterioration of Russia's monetary flexibility."

While Russia has minimal sovereign foreign debts, Russian state and private companies and banks have accumulated a total of $600 billion in foreign debts, of which around $100 billion are due next year.

The ability to repay the loans or roll them over was severely reduced this year by Western sanctions, imposed on Russia for its actions in Ukraine, which effectively shut its companies and banks out of Western debt markets.

But the economic crisis in Russia's heavily oil-dependent economy goes wider. Moody's ratings agency said late on Tuesday that it expected Russia's GDP to contract by 5.5 percent in 2015 and 3 percent in 2016, under the effect of the plunge in oil prices and the ruble's slide.

"These developments will likely lead to a severe deterioration in the operating environment for Russian corporates, namely higher inflation, unemployment and debt-servicing costs as well as lower domestic demand, resulting in a deeper and more protracted decline in domestic economic activity than previously anticipated," Moody's said.

Russia has around $414 billion in foreign exchange and gold reserves, down from around $510 billion at the start of the year, after spending heavily to prop up the ruble as oil prices almost halved from this year's peaks in June.

The central bank said Wednesday's move was aimed at “helping to refinance foreign credits by Russian exporters in foreign currencies maturing in the near future at a time of their restricted abilities to access international capital markets.”

It said these lending operations would also help to bring the ruble exchange rate into line with fundamentals and reduce volatility. Loans are to be provided for up to one year at auctions at a minimal rate of Libor plus 0.75 percent.

Oleg Kouzmin, an economist at Renaissance Capital, said the amount allocated to the instrument was small enough not to undermine the central bank's ability to support the ruble.

"Russian banks and Russian corporates will have more ability now to get dollars from the central bank to cope with external debt redemptions," he said.