US presidential contenders Elizabeth Warren and Bernie Sanders called each other liars in a heated exchange that could be seen but not heard unfolding onstage after the Democratic debate, CNN revealed on Wednesday.The spat between two of the primary campaign's most progressive standard-bearers, played out in the moments following the end of Tuesday's debate, when the majority of the six candidates cordially shook hands after the forum.A visibly irritated Warren, however, refused to take Sander's hand, appearing to rebuke him as the two senators seemed to exchange angry words.For months, Sanders and Warren have battled peacefully for the right to wave the campaign's progressive flag. But their non-aggression pact unraveled in recent days, with Warren endorsing a report that Sanders privately told her he believed a woman could not defeat Trump."I didn't say it," Sanders, 78, insisted at the debate, stressing it was absurd for anyone to think a woman could not win the White House.Warren, 70, said he did, before insisting she was "not here to try to fight with Bernie."The backup audio recordings recovered by CNN revealed they continued their exchange on the topic in the moments after the debate, lobbing accusations of "liar" at each other."I think you called me a liar on national TV," Warren said."What?" Sanders replied. "I think you called me a liar on national TV," she repeated. "You know, let's not do it right now. If you want to have that discussion, we'll have that discussion," Sanders said, as Warren replied, "Anytime." "You called me a liar," Sanders continued. "You told me - all right, let's not do it now."With no candidate yet to carve out a clear lead less than three weeks to go before the first votes in the nominations battle, the stakes in the debate were high.Iowa caucuses on February 3 will begin the presidential primary season, as the candidates battle for the right to challenge incumbent Donald Trump in November's election.