Stock index futures soared on Wednesday as investors welcomed a coordinated action by major central banks to provide liquidity to the global financial system.

The Federal Reserve, the European Central Bank as well as the central banks of Canada, Britain, Japan and Switzerland agreed to lower the cost of existing dollar swap lines by 50 basis points starting from December 5.

The actions came as China unexpectedly cut its banks' reserve requirements in hopes of boosting an economy running at its weakest pace since 2009.

S&P 500 futures jumped 33.2 points and were above fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures gained 269 points, and Nasdaq 100 futures added 58 points.

People were expecting China to do something before the end of the year, and given the stresses in the market there has been talk about the Fed backstopping what's going on in Europe. Desperate times and all. said Sal Catrini, a managing director for equities at Cantor Fitzgerald & Co in New York.

The move in (U.S. stock) futures is justified. Whether this solves our long-term problems remains to be seen, but when you flood the market with liquidity, risk assets go much higher.

Further boosting sentiment, the pace of job growth in the private sector accelerated in November, with U.S. employers adding 206,000 jobs, the ADP National Employment Report showed, topping forecasts.

Financial stocks extended gains after the central banks' action. Bank of America Corp shares rose 4.9 percent to $5.33 in premarket trading and JPMorgan Chase & Co added 5.9 percent to $30.19. Citigroup put on 6.4 percent to $26.84.

Still, financial shares could be pressured after Standard & Poor's on Tuesday reduced its credit ratings on 15 big banks, mostly in Europe and the United States as part of a sweeping overhaul of its ratings criteria. JPMorgan Chase, Bank of America, Citigroup, Wells Fargo & Co , Goldman Sachs Group Inc , Morgan Stanley each had their rating reduced by one notch each.

The pending home sales index, to be released at 10 a.m. EST, is seen rising 1.5 percent for October.

The Dow and S&P 500 advanced for a second day on Tuesday as stronger-than-expected consumer confidence data and hopes for further progress on a solution to Europe's fiscal mess bolstered sentiment.

(Reporting by Angela Moon; editing by Jeffrey Benkoe)