Warren's statement Monday evening came after CNN first reported on the private interaction. It is the latest salvo in a quickly escalating feud between the two progressive senators as voting commences in less than three weeks. It is also another sign that the non-aggression pact between the two has been irrevocably breached.

A top Sanders surrogate, Shaun King, tweeted immediately after Warren's statement that he had sources inside Warren's campaign who told him she “embellishes” stories. The Warren campaign did not respond to his comments.

The Sanders campaign also did not immediately respond to Warren’s statement. Earlier in the day, Sanders said in a statement that “staff who weren't in the room are lying about what happened.”

"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,” Sanders said. He acknowledged that gender came up during their conversation. “What I did say that night was that Donald Trump is a sexist, a racist and a liar who would weaponize whatever he could," Sanders said.

Sanders campaign manager Faiz Shakir went further. "We need to hear from her directly, but I know what she would say: That it is not true, that is a lie. I welcome her coming out and disputing this and putting this to rest," he told CNN.

Warren's statement made clear that Shakir was mistaken. Still, the Massachusetts senator, after confirming CNN's initial report, insisted she wanted to move on. “I have no interest in discussing this private meeting any further because Bernie and I have far more in common than our differences on punditry,” she said in her statement.

The clash alarmed some leaders of the progressive movement, who said that a prolonged bout between the two could damage them both.

The cofounders of Progressive Change Campaign Committee, a close ally of Warren’s campaign, issued a statement before Warren’s that also seemed to call for de-escalation. A "back-and-forth about this private meeting is counter-productive for progressives,” the group said, while adding "that sexism is real and it has played a huge role in this campaign. We also believe that women can win and will win in 2020.”

But it may be too late: The two are set to face off Tuesday night at a debate in Des Moines. They are sure to be asked about what was said during the meeting, and who's version is accurate.

Over the weekend, POLITICO reported that the Sanders campaign had produced a script instructing volunteers to tell Warren-leaning voters that “people who support her are highly educated, more affluent people who are going to show up and vote Democratic no matter what” and that “she's bringing no new bases into the Democratic Party.”

The report drew a tough response from Warren, who said she was “disappointed” Sanders was “sending his volunteers out to trash me” and that he risked dividing the party with attacks against her and others.

“We all saw the impact of the factionalism in 2016, and we can’t have a repeat of that,” she said, suggesting that Sanders’ campaign against Hillary Clinton hurt her in the general election. “Democrats need to unite our party.”

Even so, some other prominent progressives also called on Monday for a detente, fearing that a battle between the two top liberal candidates would hurt the broader movement. “This looks like a desperate attempt to fracture a coalition of the candidates that represent the most popular ideas among working people,” said Sara Nelson, president of the Association of Flight Attendants union.

And Democracy for America tweeted that “@BernieSanders and @ewarren, you both are progressive champs & our movement needs to see you working together to defeat your corporate Dem opponents — not attack each other.”

Holly Otterbein contributed to this report.