TEHRAN, Iran — Iran has discovered an oil field in the country’s south with more than 50 billion barrels of crude oil, its president said on Sunday, a development that could bolster the nation’s proven reserves by a third as it struggles to sell energy abroad.

The announcement by President Hassan Rouhani comes as Iran faces crushing American sanctions imposed last year when the United States withdrew from nuclear deal with Iran and world powers.

Mr. Rouhani, who made the announcement in a speech in the desert city of Yazd, said the field was in the southern province of Khuzestan, home to its crucial oil industry. Some 53 billion barrels would be added to Iran’s proven reserves of 150 billion, he said.

“I am telling the White House that in the days when you sanctioned the sale of Iranian oil, the country’s workers and engineers were able to discover 53 billion barrels of oil,” Mr. Rouhani said, according to the semiofficial Fars news agency.