NEW YORK (Reuters) -U.S. support for Argentina's markets has prevented a downgrade of the South American country's credit rating, but the country needs a broader plan to rebuild foreign exchange reserves in order to earn an upgrade, Fitch Ratings said on Wednesday.

"The U.S. backstop is something that has helped Argentina ward off a ratings downgrade, the risks of which would have risen if the central bank had kept bleeding international reserves to defend the FX regime," Todd Martinez, co-head of the Americas for Fitch Ratings' sovereigns group, said in an email.

Argentina's central bank said this week it signed a $20 billion exchange-rate stabilization agreement with the U.S. Treasury Department, part of a U.S. plan to support President Javier Milei's reform policies ahead of key midterm elections this weekend.