A deal could be announced as soon as Thursday, it said.
But a greater U.S. stake will bring a slew of new complications for executives of the New York company, the report said.
Mexican law bars any institution more than 10 percent-owned by a foreign government from running a bank in that country. Some Citigroup executives are worried that an increased U.S. stake might subject the bank to pressure to relinquish some or all of its ownership of Grupo Financiero Banamex, the No. 2 bank in Mexico by assets, the Journal said.
Though Citigroup is loath to shed Banamex, seen as a crown jewel of the bank's operations, executives have concluded that the issue will probably have to be resolved through diplomatic channels between the United States and Mexico, people familiar with the matter said, according to the report.
A representative of Citigroup was not immediately available for comment.
(Reporting by Euan Rocha; Editing by Gary Hill)