ATLANTA — 7-Eleven Inc., the largest American operator of convenience stores, said it will start selling gasoline under its own brand after a 20-year relationship with Citgo Petroleum Corp. ends this week.

7-Eleven’s agreement to buy most of its fuel from Citgo, the American subsidiary of Venezuela’s state-run oil company, expires September 30. 7-Eleven has been planning to end the contract since last year because Citgo said it would stop providing gasoline in Texas, where the retailer operates “hundreds of stores,” a 7-Eleven spokeswoman, Margaret Chabris, said in an e-mail.

The contract is ending amid rising tensions between the Venezuelan and American governments.

President Chavez of Venezuela last week criticized American foreign policy and the war in Iraq, calling President Bush “the devil” and a “world tyrant” in a speech to the U.N. General Assembly.

“We knew our contract with Citgo would be ending at midnight on September 30,” Ms. Chabris said. “Certainly, Chavez’s position and statements over the past year or so didn’t tempt us to stay with Citgo.”

7-Eleven will start selling gasoline under its own brand with supplies from Frontier Oil Corp., Tower Energy Group, in Torrance, Calif., and Sinclair Oil of Salt Lake City. The retailer said in a March 2005 filing with the U.S. Securities and Exchange Commission it was considering other fuel-suppliers once its Citgo agreement ended.

The removal of the Citgo signs will be gradual, the company said. They will be removed from Dallas-based 7-Eleven’s more than 2,100 locations by 2008. 7-Eleven was taken private by its Japanese parent company Seven & I Holdings Co. in November 2005.