Republican US presidential front- runner Donald Trump has said if elected he would prosecute his Democratic rival Hillary Clinton for using a private email server when she was the secretary of state.

"Well look, you have no choice. I mean we have to solve all sorts of problems, and fairness is you have to look into that. Now, maybe she can prove her innocence, but it just seems to me I think the public knows everything that they're going to know," Trump told Fox News in an interview.

"You know; the emails have come out. And she seems to be guilty. But you know what, I wouldn't even say that. But certainly it has to be looked at if a Republican win, if I'm winning. Certainly you will look at that as being fair to everyone else," Trump said in response to a question.

"It's so unfair to the people that have been prosecuted over the years for doing much less than she did. So she's being protected. But if I win, certainly it's something we'll want," the New York billionaire said.

Clinton, however, fired back quickly claiming nothing has been proved so far against her.

"I can only tell you what the facts are, and the facts are that every single time somebody has hurled these charges against me, which they have done, it's proved to be nothing. And, this is no different than that,” Clinton said at a CNN-organised town hall in Columbia, South Carolina, which goes for the Democratic presidential primary over the weekend.

The former Secretary of State was responding to questions on a ruling by a federal judge on a motion that could pave the way for the possibility that you could be subpoenaed in order to obtain any information.

Criticised widely for saying last week that "she did not believe she ever lied", Clinton said she would "just say no" if asked again if she had ever lied to the American people.

She rued she is being put to different standard than any other candidate."Why is there one standard for me and not for everybody else?" Clinton asked.

The former Secretary of State is leading South Carolina by a huge margin and is expected to win the Democratic party primary over the weekend.

Clinton argued that the allegations by the opposition that just because she has taken donations from the Wall Street so she will not take action against them, does not hold good.

"I am on record...I went to Wall Street before the Great Recession. I called them out. I said what they were doing in the mortgage market was going to cause serious problems," she asserted.