(Reuters) - Standard & Poor's Ratings Services on Friday revised its outlook on some Chicago O'Hare International Airport bonds to stable from positive over concerns about a $2.3 billion modernization program and broader economic uncertainties.

The credit rating action affects third-lien general airport revenue bonds and stand-alone passenger facility charge bonds, both of which are rated 'A-minus.'

Firm financial plans for the modernization project "are not likely to be finalized until 2014 or later," said S&P credit analyst Joseph Pezzimenti in a statement on the reasons for the change in outlook.

S&P also said that the sluggish economy and volatile fuel prices could stagnate the number of passenger boardings and limit revenue from passenger facility charges.

There is also uncertainty over what form American Airlines Inc. will take once the company exits bankruptcy, S&P said.

The airport could see bond ratings cut in the next two years if costs for the modernization project aren't contained, if the project disrupts operations or if the number of passengers boarding planes drops significantly, S&P said.

(Reporting By Hilary Russ; Editing by Will Dunham and Diane Craft)