Banco Santander (SAN.MC) filed to list a portion of its Mexican unit, Santander Mexico Financial Group, in a U.S. initial public offering of up to $100 million as the Spanish bank looks to shore up its finances to tackle economic uncertainty at home.

Reuters publication IFR reported on Thursday that the bank was looking to spin-off its Mexican unit in an offering that could range between $3 billion and $4 billion.

Banco Santander had said, in a separate filing with Mexico's stock exchange on Thursday, that it intends to list up to 25 percent of the unit, with just over 6 percent being sold in Mexico and 18.7 percent abroad.

As of Friday, Banco Santander owned about 99.87 percent of the Mexican unit's capital stock, according to the filing.

Banco Santander and its Santusa Holding unit will be selling the American depositary shares in the offering, with each ADS representing five shares of Santander Mexico's Series B common stock.

Concurrently with the U.S. offering, Banco Santander is also offering Series B shares through a public offering in Mexico, the filing revealed.

Santander Investment Securities, UBS Investment Bank, Deutsche Bank Securities and BofA Merrill Lynch are acting as lead underwriters to the IPO.

Santander Mexico intends to list its ADSs on the New York Stock Exchange under the symbol "BSMX", the filing said.

Banco Santander, the Euro zone's biggest bank, is battling recession at home and a debt crisis that is spreading across the common currency bloc.

Its Latin American operations -- in Argentina, Brazil, Mexico, Chile, Peru, Puerto Rico and Uruguay -- make up nearly a fifth of the group's assets and 52 percent of its net profit.

The amount of money a company says it plans to raise in its first IPO filings is used to calculate registration fees. The final size of the IPO could be different.