Tara Reade, who worked as a staff assistant in Biden's US Senate office from December 1992 to August 1993, has accused Biden of pinning her against a wall in 1993 and reaching under her shirt and her skirt.

Biden's campaign has said the incident never occurred. Biden himself has not addressed the allegation, and the former vice president has not been questioned on the matter in recent interviews with national and local media.

Two more women corroborated part of Reade's allegation in interviews with Business Insider.

A Biden campaign spokesman declined to comment on Wednesday about the new sources or calls for Biden to respond.

Reuters has not been able to independently confirm Reade's allegation and was also unable to reach Reade or a representative for comment.

The New York Times said in a statement on Wednesday an investigation it conducted of the matter "made no conclusion either way".

US Representative Alexandria Ocasio-Cortez, an influential liberal Democrat, said during an online forum this week that Reade's accusation deserves a fair examination.

Nina Turner, who was national campaign co-chair for Biden's former White House rival, Bernie Sanders, told Reuters on Wednesday that Biden has to answer the question.

"It can't appear that she's being ignored just because it's an inconvenient truth for certain people in the Democratic Party," Turner said.

The Washington Post editorial board on Wednesday called on Biden, 77, to speak about the allegations and release papers from the relevant period, when he was a senator from Delaware, in case those documents might include a complaint Reade has said she filed over the incident.

Several prominent women on Biden's vice-presidential short list, such as Senators Kamala Harris and Amy Klobuchar and former Georgia state Representative Stacey Abrams, have stood by Biden.

So has Senator Kirsten Gillibrand, another former presidential candidate, who has made aiding victims of sexual assault a centerpiece of her political career.

President Donald Trump's re-election campaign has tried to turn the allegations into a liability for Biden.

In a memo to the media on Wednesday, Trump campaign spokesman Tim Murtaugh said Biden was seeking to shield himself from criticism by avoiding personally addressing Reade's allegation.

Trump, 73, a Republican who will face Biden in the November 3 general election, has been accused by more than a dozen women of sexual assault and misconduct over the years, allegations which he and his aides have consistently denied.

Reade, 56, was one of eight women who last year came forward to say Biden had hugged, kissed or touched them in ways that made them uncomfortable, though none accused him of sexual assault. She publicly accused him of the assault on a podcast in March.

At the time of the women's statements last year about behaviour that made them uncomfortable, Biden, who had yet to launch his presidential bid, released a video promising to be "much more mindful" of people's personal space.

Australian Associated Press