(CNN) — South Carolina Gov. Mark Sanford admitted Wednesday, amid speculation over his whereabouts for the last several days, that he has been engaged in an extramarital affair with an Argentinian woman.

“I’ve been unfaithful to my wife,” the Republican governor told a news conference in Columbia, the state capital. “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 was discovered five months ago, Sanford said.

The South Carolina governor had not been seen in public since June 18. When questioned, Sanford’s staff had told media outlets that he was hiking in the Appalachian Trail. But Sanford was spotted Wednesday in Hartsfield-Jackson International Airport in Atlanta, Georgia. Watch Sanford admit to the affair »

The South Carolina governor said he had been in Buenos Aires, Argentina. He said it was his fault for “shrouding this larger trip.”

“I’m a bottom-line kind of guy. I’ll lay it out, it’s going to hurt, and we’ll let the chips fall as they may,” Sanford said as he spoke with reporters.

Sanford said his affair began as a friendship, but “over this last year, it developed into something much more than that.”

“All I can say is that I apologize,” he said, adding that he would appreciate a “zone of privacy” for the sake of his family.

When asked if he is trying to reconcile with his wife, Jenny, Sanford nodded, “I am, yeah.” Recent political sex scandals »

Pressed further on whether he and his wife are separated, he said, “I don’t know how you want to define that. I mean, I’m here and she’s there. I guess in a formal sense we’re not, but what we’re trying to do is work through something that we’ve been working through for a number of months now.”

Sanford, 49, is a conservative Republican mentioned as a possible presidential candidate in 2012. He is serving his second straight term as governor, which expires in January 2011. The state constitution prohibits a third straight term.

Mississippi Gov. Haley Barbour immediately takes over as head of the Republican Governors Association, CNN has learned.

“The news revealed today hurts all of us who have gotten to know Governor Sanford over the years and so it is with regret that the RGA accepted Governor Sanford’s resignation as chairman,” Barbour said in a statement obtained by CNN that will be distributed Wednesday afternoon. “While this news is deeply disappointing, I also know it’s important to remain focused on the future and Governor Sanford’s resignation allows him and us to do just that.”

Shortly after Sanford’s announcement, Democratic Governors Association Executive Director Nathan Daschle issued a statement saying, “Our thoughts and prayers are with Gov. Sanford and his family, and I wish them the best as they try to heal this wound.”

The South Carolina governor has had a bruising year, losing his fight to turn down $700 million in federal stimulus money. Sanford initially tried to require that any stimulus money pay down the state debt, a stance rejected by the Obama administration.

The South Carolina’s Supreme Court ruled earlier this month that Sanford had to accept the stimulus funds.

Earlier Wednesday, South Carolina Democratic Party Chairwoman Carol Fowler said in a statement it was the “height of irresponsibility for the governor to walk out on his solemn duty to the people of South Carolina.

“When it comes to the real needs and concerns of South Carolinians, Mark Sanford has been pretty much missing in action for the last six-and-a-half years. But it is not acceptable for him to be actually missing.

“However, as disturbing as it is to walk out on your responsibilities without telling anyone, what’s more troubling is that the governor and his staff have been less than truthful about his whereabouts,” the statement said.

Sanford’s wife Jenny told CNN on Tuesday she had not talked to her husband since he left last Thursday and did not know where he went.

“I am being a mom today. I have not heard from my husband. I am taking care of my children,” she said outside the couple’s beach home on Sullivan’s Island, South Carolina. – CNN