Bernie Sanders has denied telling Elizabeth Warren, his rival in the race to become the Democrats' candidate in the US presidential election, he did not think a woman could win the White House.

Key points: Bernie Sanders has rejected the allegation, saying he instead said Mr Trump was a sexist who would weaponise "whatever he could"

Bernie Sanders has rejected the allegation, saying he instead said Mr Trump was a sexist who would weaponise "whatever he could" He and fellow Democratic candidate Elizabeth Warren have in the past described themselves as friends

He and fellow Democratic candidate Elizabeth Warren have in the past described themselves as friends The Democratic primary has until now been characterised by genial differences

On Monday Senator Warren said Senator Sanders made the claim when they met privately in 2018.

Senator Warren said in a statement that during their two-hour meeting to discuss the 2020 election, "among the topics that came up was what would happen if Democrats nominated a female candidate. I thought a woman could win. He disagreed".

That drew a swift and strong denial from the Vermont senator.

"It is ludicrous to believe that at the same meeting where Elizabeth Warren told me she was going to run for president, I would tell her that a woman couldn't win," he said.

"What I did say that night was that [President] Donald Trump is a sexist, a racist and a liar who would weaponise whatever he could."

The dispute marks a turning point in a Democratic primary that, with few exceptions, has been characterised by genial differences over domestic issues such as health care.

The pair previously said they were friends and would not turn on each other.

CNN first reported Senator Sanders's alleged comments earlier on Monday, based on the accounts of anonymous people with knowledge of the meeting.

Senator Sanders's aides then accused Senator Warren's campaign of leaking what they said was an inaccurate description of what was said during the meeting.

Tensions between the pair grew over the weekend following a Politico story which reported the Sanders campaign had instructed some volunteers to characterise Senator Warren as a candidate for wealthy and well-educated voters in conversations with undecided voters, Senator Warren issued a rare critique of her opponent.

She said she was "disappointed" Senator Sanders was instructing staffers to "trash" her.

Senator Sanders said he had not approved the negative talking points related to his Democratic rivals.

Elizabeth Warren has enjoyed a steady rise in standing within the Democratic party. ( AP: Michael Dwyer )

Senators Warren and Sanders are among six candidates who will face off on Tuesday in the seventh Democratic presidential debate, the last before the February 3 Iowa caucuses, which kick off the state nominating contests.

There are 12 candidates currently vying for the party's nomination to take on Mr Trump in November.

National opinion polls show Senator Warren in the top tier but trailing Senator Sanders and former vice-president Joe Biden.

AP/ABC