Sen. Bernie Sanders reportedly told Sen. Elizabeth Warren that "he does not believe that a woman can win" the presidency during a private meeting between the two in 2018.

Four sources claimed that Sanders issued the statement during a meeting at Warren's Washington apartment. Warren allegedly disagreed with the claim, and her campaign did not respond to questions on Monday about the incident.

Sanders's campaign dismissed the claims as "lies" and said it was "sad" that the allegations had come out only weeks before the first-in-the-nation primary vote in Iowa.

"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. It's sad that three weeks before the Iowa caucus and a year after that private conversation, staff who weren't in the room are lying about what happened," said the Sanders campaign in a statement.

Sanders, 77, and Warren, 70, have feuded over the past few days after a leaked internal memo from Sander's campaign showed he instructed volunteers to attack Warren for having an "affluent" voter base. Warren told reporters in Iowa that Sanders was trying to "trash" her.

[Read: 'No one is going to be attacking Elizabeth': Sanders distances himself from leaked campaign memo]