The announcement, which the ratings agency released Tuesday afternoon, cited a recent Illinois court ruling voiding state pension reforms. Moody's said it saw a negative outlook for the city's credit.

Following that May court decision, Moody's said it believes that "the city's options for curbing growth in its own unfunded pension liabilities have narrowed considerably."

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"Whether or not the current statutes that govern Chicago's pension plans stand, we expect the costs of servicing Chicago's unfunded liabilities will grow, placing significant strain on the city's financial operations absent commensurate growth in revenue and/or reductions in other expenditures," the agency said in a release.