Mexico's Carlos Slim, the world's richest man, has acquired an 8.4 percent stake in YPF, the recently nationalized Argentine subsidiary of Spanish oil firm Repsol, filings show.
Ratings agency Moody's had downgraded YPF another notch on Tuesday, a month after Buenos Aires seized control of Argentina's largest oil firm that accounts for a third of the country's domestic production.
A US Securities and Exchange Commission filing on Thursday showed that Slim's investment company, Domitgrupo Financiero Inbursa, had acquired almost 33 million shares in YPF, which is listed on the New York Stock Exchange.
YPF CEO Miguel Galuccio hailed the purchase, saying it would send a "clear signal to international markets" and was a "show of confidence in Argentina and the new plans for the company."
However, Slim's spokesman Renato Flores told AFP that his company had not purchased the shares directly but acquired them after they were posted as collateral on a loan that went into default.
On Tuesday Moody's downgraded its rating for YPF to Caa1 from B3, citing concerns over the company's debt but saying its outlook was stable.
Argentina declared YPF a public utility on April 16, with the government seizing a 51 percent stake in the Repsol subsidiary -- infuriating Madrid and sparking concern in the United States and the European Union.
Later the same week, it extended the move to YPF Gas, a separate company 85 percent owned by a division of Repsol.
President Cristina Kirchner has argued that the expropriation was justified because YPF crude production had dropped while oil and gas imports doubled in 2011. Imports are forecast to triple by the end of the year.
Slim, a 72-year-old Mexican tycoon who built his fortune in the telecoms industry, is the richest man in the world with an estimated $69 billion fortune, according to Forbes magazine.