A woman has made sexual harassment allegations against Republican US presidential hopeful Herman Cain, telling a news conference that he attempted to inappropriately touch her in 1997.

Sharon Bialek releaved in New York on Monday that she met Cain to ask about getting her old job back at the National Restaurant Association when the incident happened in Washington.

At the time, Cain was chief of a restaurant trade group. Bialek said Cain reached under her skirt and reached for her genitals after she had asked for his help.

When she asked Cain what he was doing, Cain allegedly said, "You want a job, right?"

Bialek appeared with her attorney, Gloria Allred, at a news conference in New York.

Swift denial

A spokesman for Cain's campaign team immediately issued a denial, saying that "all allegations of harassment against Mr Cain are completely false".

He said Cain had "never harassed anyone" and derided the allegations as "bogus attacks".

At least three other women have accused Cain of sexual harassment from his time as the restaurant industry's top lobbyist in the mid-1990s, but Bialek is the first one to go public.

Cain, 65, insists the claims are baseless and that he was wrongfully accused.

The former pizza company executive with no experience in political office, has been leading many opinion polls in the race to become the Republican presidential nominee to face Barack Obama in next year's presidential election.

The latest development comes just two days before Republican candidates gather in Rochester, Michigan, on Wednesday night for the latest in a series of campaign debates.

Presidential aspirations

A Reuters/Ipsos poll released on Sunday showed the percentage of Republicans who viewed Cain favourably had dropped nine percentage points, to 57 per cent from 66 per cent a week ago.

The survey was the first evidence that sexual harassment claims against Cain have taken a toll on his presidential aspirations.

A woman who received a cash settlement from the restaurant association in response to her harassment claim rejected Cain's denials on Friday.

She said through her lawyer she was the victim of a "series of inappropriate behaviours and unwanted advances" by Cain in 1999.

One of Cain's Republican presidential adversaries called on Sunday for more information on the allegations.

"It has got to come out in total," Republican presidential hopeful Jon Huntsman told NBC's "Meet the Press" on Sunday. "Legitimate questions have been raised and that information has to come forward."