(CNN) — South Carolina Gov. Mark Sanford, just back from a secret trip to Argentina unknown to his staff or his wife, admitted Wednesday he has carried on an extramarital affair with a woman in Buenos Aires, Argentina.

“I’ve been unfaithful to my wife,” Sanford told a news conference in Columbia, South Carolina. “I developed a relationship with what started as a dear, dear friend from Argentina.”

His voice choking at times, Sanford apologized to his wife and four sons, his staff and supporters, and said he would resign immediately as head of the Republican Governors Association.

The affair began in the last year and was discovered five months ago, Sanford said without elaborating. He added that he and his wife were trying to work through it.