Republican South Carolina Governor Mark Sanford has admitted to having an affair with a woman he visited during a secret trip to Argentina. The revelation casts doubt on Sanford's hopes of running for president in 2012.

Governor Mark Sanford made the announcement in the state capital, Columbia,Wednesday after returning from an unannounced trip early this week to Argentina. State officials said they did not know where Sanford was for several days, and said his top aides claimed he was hiking the nearby Appalachian trail. 

After returning, Sanford said he actually went to Argentina to visit the woman he had started seeing romantically about a year ago.

"So the bottom line is this: I have been unfaithful to my wife," said Sanford. "I developed a relationship with what started as a dear friend from Argentina."

Sanford did not name the woman, but said the two started a friendship through email and grew close as he counseled her on trying to repair her relationship with the father of her children.

"Recently over this past year it developed into something much more than that and as a consequence I hurt her, I hurt you all, I hurt my wife, I hurt my boys," he said.

The Republican governor said he admitted the affair to his wife and four children about five months ago, but did not tell her about the trip to Argentina.

The 49-year-old Governor said he told his staff that he had intended to vacation on the Appalachian trail, but that he later changed his travel plans and did not inform them. He said his actions had damaged trust in the state, and he promised to work to regain people's confidence.

"So that means me going one by one and town by town, to talk to a lot of old friends across the state about what I've done and asking for their forgiveness," said Sanford.

Sanford said he was resigning as head of the Republican Governors Association to spend more time in the state. He did not answer questions about whether he planned to resign as governor.

Sanford has about a year remaining in his second term, and is barred from running again. He had won praise for his conservative economic policies, and some Republican party leaders had named Sanford as a top choice to run for president in 2012.

Last week, Republican Senator John Ensign of Nevada also revealed that he had an affair with a campaign staffer. He resigned from a Republican party position but has not said he will quit the Senate.