Warren, Sanders have heated off-mic moment after debate

US Presidential candidates Elizabeth Warren and Bernie Sanders appear to have fallen out in the crucial weeks before the Iowa caucus, if an interaction immediately after today's debate is any indication.

Senator Warren appeared to snub a handshake from Senator Sanders before the pair tensely exchanged words.

Senator Sanders wagged his finger at Senator Warren before they both turned their backs on each other.

Candidate Tom Steyer became awkwardly caught in the middle briefly.

"Oof," said a CNN commentator during the moment, though audio of their interaction was not picked up.

It was a striking confrontation on the stage after the debate, especially given the pair have a reputation as friends.

The two have effectively accused each other of lying about a meeting they had in 2018, as they were both readying presidential bids.

Senator Warren alleged that Senator Sanders told her a woman could not be elected president. Senator Sanders denied the claims.

"As a matter of fact, I didn't say that," he said in the debate today.

"In 2015 I deferred to Senator Warren. There was a movement to draft her. She decided not to run, so I did run."

Senator Sanders noted that his 2016 rival, Hillary Clinton, won the popular vote over Donald Trump by three million votes.

"Does anybody in their right mind believe a woman cannot be president?" he said.

"Who believes that a woman can't win? Of course a woman can win!"

Senator Warren said she was "not here to try and fight with Bernie".

"Look at the men on this stage," she said.

"Collectively they have lost ten elections. The only people on this stage who have won every single election that they've been in are the women."

The crowd cheered at her remark.

As progressives with similar policy platforms, Senator Sanders and Senator Warren have been competing for the same voters in the Democratic primary.

Senator Sanders has been edging ahead in the polls, but Senator Warren remains in striking distance three weeks from the crucial Iowa caucus.

But the frontrunner remains former Vice President Joe Biden, who left the debate relatively unscathed, with few seeking to criticise him directly.