BUENOS AIRES/MEXICO CITY (Reuters) - Mexican tycoon Carlos Slim and his family have taken a stake in Argentina's recently renationalized energy company YPF (>> YPF SA) in lieu of a loan guarantee, but they have no plans to increase it for now, his spokesman said.

YPF said in a statement that the Slims had acquired 8.4 percent of the company's shares, in what it called a "long-term investment." A filing with the U.S. Securities and Exchange Commission had specified that the Slims had acquired 8.4 percent of Class D shares.

A YPF spokesman later confirmed that the stake bought by the Slims amounted to 8.4 percent of the entire company.

Argentine President Cristina Fernandez seized control of YPF from Repsol (>> Repsol YPF SA) in April, accusing the Spanish oil company of investing too little and making the country increasingly reliant on pricey imports.

That move slammed YPF's stock price and was met by howls from European trade partners and investors.

The SEC filing said the Slim family's Inmobiliaria Carso and Grupo Financiero Inbursa held the shares.

YPF said in a statement that the Slims' acquisition of the stake was a vote of confidence in the Argentine company and "a clear signal to international financial markets."

Slim's spokesman, Arturo Elias Ayub, told Reuters the Slims did not plan to increase their stake in YPF for now.

"We think the company is solid and has good growth potential," he said. "In time, we will analyze our options."

YPF's stock price hit 10-year lows earlier this week after the company said on June 5 that it would need to invest $7 billion a year to boost flagging natural gas and oil output by more than a quarter by 2017.

A source close to the situation told Reuters that Slim had acquired the shares from banks that had helped Argentina's Petersen Group buy a 25.46 percent stake in YPF.

This week, banks including Credit Suisse (>> Credit Suisse Group AG), Citi (>> Citigroup Inc.), Itau (>> Itau Unibanco Holding SA) and BNP Paribas (>> BNP PARIBAS) received Petersen's YPF shares after the company failed to repay loans.

The loans were backed by YPF shares.

Argentina's government said last year that Slim, the world's richest man, would invest $1 billion in the South American country by the end of 2012, mainly in the telecommunications sector.

His America Movil (>> America Movil SAB de CV) is one of the world's biggest cellphone providers, with 225 million wireless customers in countries stretching from the United States to Argentina.

YPF Chief Executive Officer Miguel Galuccio, a former executive at global oilfield services company Schlumberger (>> Schlumberger Limited.), said YPF would be looking for deep-pocketed partners to help fund its plans to reverse Argentina's energy deficit.

Fitch Ratings cut Repsol's credit rating last week by one notch to "BBB-minus," citing the expectation that the company would receive no compensation in the near or medium term from Argentina for the expropriation of its majority stake in YPF.

(Writing by Hugh Bronstein; Editing by Simon Gardner and Gary Hill)

By Guido Nejamkis and Cyntia Barrera