"A resolution with the Department of Justice could include a guilty plea on an antitrust charge," it said.
The bank also said in a statement the regulator will not prosecute the bank relating to its Libor dealings.
Citigroup is among six large banks being probed by global authorities for allegedly trying to manipulate exchange rates in currency markets.
The lender reported it was being sued by Argentina and that some employees could face criminal charges there as the country does legal battle with US hedge funds.
Sanctions and "business restrictions" could result, Citigroup said: "The situation could also expose Citi and Citi Argentina to further litigation and penalties."
US hedge funds NML Capital and Aurelius Capital Management are seeking $1.3bn (??834m) after refusing to accept a restructuring deal on Argentine debt.
Argentina's move comes after US District Judge Thomas Griesa's ruling in March that Citigroup cannot process interest payments by Argentina on some bonds issued under that country's law.
Mr Griesa ruled last year that other holders of Argentina's debt - many of whom agreed to Argentina's restructure -  should not receive payments until the hedge funds are paid.
Argentina refused to pay the hedge funds, sending the country into default.
Shares in Citigroup were flat at $53.99 in afternoon trading in New York.
Citigroup said it may plead guilty amid a US Department of Justice probe into its foreign exchange business.