Mexican billionaire Carlos Slim, who already controls America Movil, tightened his grip on Latin America's biggest phone company, according to a U.S. regulatory filing on Thursday that shows two companies controlled by Slim bought more shares.

Slim's real estate company Inmobiliaria Carso and his bank Grupo Financiero Inbursa spent $212.5 million and $34.7 million purchasing 187.11 million and 30.98 million shares respectively in America Movil, a document sent to the U.S. Securities and Exchange Commission showed.

It was not immediately clear what percentage of America Movil is now held by the two companies, Slim and his family members.

America Movil shares have been under pressure since Mexico's government last year approved a reform that will introduce more competition to the phone, internet and television business in the country.

The shares are down 10.45 percent year to date. The shares closed down 2.05 percent on Thursday at 13.35 pesos.