Elizabeth Warren accused Bernie Sanders of calling her a "liar on national TV" in a heated exchange following the televised Democratic debate, according to newly-released audio.
The exchange between the two presidential hopefuls came as Ms Warren appeared to snub Mr Sanders's offer of a handshake following the debate in Iowa on Tuesday.
Neither campaign would confirm what was said during the tense conversation, but audio of the row from CNN has now given more context to the clash.
It comes amid a spat between the rivals over Ms Warren's claims that Mr Sanders said a woman could not win the White House, a claim he strenuously denies.
As the pair left their podiums, after rowing during the debate, Mr Sanders held out his hand, as if for Ms Warren to shake it.
Refusing to reach out for his hand, she is heard saying: "I think you called me a liar on national TV."
Mr Sanders then asks "what?" before Ms Warren repeats the statement.
He then replies: "Let's not do it right now. You want to have that discussion, we'll have that discussion."
Ms Warren interjects: "Anytime."
Mr Sanders then accuses her of the same slight, continuing: "You called me a liar, you called me..."
He then concludes: "Let's not do it now."
Fellow candidate Tom Steyer then interrupted the exchange as he said: "I don't want to get in the middle, I just want to say hi, Bernie."
Ms Warren walked away as Mr Sanders said to Mr Steyer: "Yeah, good, okay."
The spat came after Ms Warren claimed Mr Sanders “disagreed” that a woman could be elected as US president in 2020.
In a statement on Monday, Ms Warren said: "Among the topics that came up was what would happen if Democrats nominated a female candidate.
"I thought a woman could win; he disagreed."
Mr Sanders has denied the accusation and did so again in the debate as he said it was "incomprehensible" he could have expressed this view.
Ms Warren confirmed her opinion on the comment and said she disagreed with Mr Sanders.
Ms Warren’s and Mr Sanders’s aides have attempted to de-escalate the feud as some progressives it could benefit more moderate Democratic presidential hopefuls such as former Vice President Joe Biden and Pete Buttigieg.
There are just three weeks until first nominating contest in Iowa, with 12 candidates still vying for the spot.
Opinion polls show the top four contenders are former Vice President Joe Biden, Mr Sanders, Ms Warren, and former mayor of South Bend, Indiana, Pete Buttigieg.
