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The rating agency also affirmed Serbia's BB-minus long-term foreign and local currency ratings.
"The negative outlook reflects the deterioration in the country's fiscal and external financing position and weak economic growth outlook," Fitch said in a statement.
"Rather than focusing on correcting a rising fiscal deficit and public debt ratio, the new government has amended the central bank law in a manner which has dented investor confidence and might complicate the agreement of a new IMF deal."
Standard & Poor's rates Serbia BB-minus with a negative outlook.