Herman Cain, the US Republican party's hopeful for the White House, has said he is reassessing his 2012 presidential bid after allegations of an extra marital affair, his campaign staff said.

"Mr Cain told staff simply that, just as every time significant events occur, a reassessment is prudent," ABC quoted his deputy campaign manager, Linda Hansen, as saying on Tuesday.

In the latest blow to Cain's campaign, a woman in Atlanta, Georgia, alleged on Monday that she had a 13-year affair with him which ended just eight months ago. Cain has acknowledged a friendship with the woman, but denied they had an affair.

Hansen reportedly was on a conference call on Tuesday morning in which Cain said he was looking at the viability of his challenge for the Republican nomination to battle President Barack Obama in the November 2012 elections.

The National Review magazine listened to and transcribed the five-minute morning conference call, in which Cain somberly acknowledged knowing the woman who claimed the affair, Ginger White, "for a number of years".

But he unequivocally denied the two had an affair, stressing it was a friendship and that he was helping her financially "because she was out of work and destitute, desperate," the magazine quoted Cain as saying.

'Cause for reassessment'

"That being said, obviously, this is cause for reassessment," Cain said, adding he would complete his assessment "over the next several days," according to the magazine transcript.

On the call the candidate described the reassessment as being similar to those made by his campaign after revelations late last month that he faced 1990s sexual harassment accusations from four women - two of whom came forward in public. He has denied all impropriety.

"Now, with this latest one, we have to do an assessment as to whether or not this is going to create too much of a cloud, in some people's minds, as to whether or not they would be able to support us going forth," Cain said according to National Review.

On Monday Cain's attorney Lin Wood sought to downplay the affair claim.

"This appears to be an accusation of private, alleged consensual conduct between adults - a subject matter which is not a proper subject of inquiry by the media or the public," Wood said.