Rudy Giuliani called Michael Cohen a "pathological liar" in a heated CNN interview on Thursday.

It came after multiple outlets reported that Cohen had claimed that President Donald Trump approved the June 2016 Trump Tower meeting between his campaign staff members and Russians.

Trump has consistently denied that he knew about the meeting, and he did so again on Friday.

Giuliani's calling Cohen a "pathological liar" would suggest Cohen lied all the time, presumably including when he worked for Trump in the past.

That would create another headache for Trump's defense team.

Giuliani went for Cohen during the interview, calling him "an incredible liar who's got a tremendous motive to lie."

Rudy Giuliani called Michael Cohen a "pathological liar" on Thursday in a heated response to reports that Cohen said President Donald Trump personally approved the June 2016 meeting between members of his campaign staff and representatives of Russia.

But the fierce criticism could create a new legal problem for Trump, given that Cohen worked as his lawyer for 11 years.

Earlier Thursday, multiple outlets reported that Cohen claimed he was there when Donald Trump Jr. informed Trump of a Russian offer to share dirt on Hillary Clinton, the 2016 Democratic presidential nominee.

Cohen said Trump then approved of going ahead with the meeting, CNN reported on Thursday night, citing sources with knowledge of the matter.

CNN's sources said Cohen is willing to make those claims to Robert Mueller, the special counsel investigating the Trump campaign's ties to Russia.

Giuliani, Trump's defense lawyer, launched a barrage of personal attacks on Cohen in an interview with CNN later Thursday, calling him a "pathological liar" and "an incredible liar who's got a tremendous motive to lie now because he's got nothing to give."

You can watch Giuliani's comments here:

—Cuomo Prime Time (@CuomoPrimeTime) July 27, 2018

"I expected something like this from Cohen," Giuliani told CNN's Chris Cuomo. "He's been lying all week, or for two weeks — he's been lying for years. The tapes that we have demonstrate any number of very serious lies by him back a year and a half ago."

Giuliani later added: "There's no doubt in my mind that he's just not credible. I would not accept him as a witness as a prosecutor ... This is the kind of witness that can really destroy your whole case, because any finder of fact loses confidence in the case when you rely on a guy like this. When you rely on a guy where before you're finished with him you're going to have such a string of lies, you just can't trust him."

But in calling Cohen a pathological liar, Giuliani may have created another legal problem for Trump by effectively asserting that Cohen lied for Trump while he worked for him, from 2007 — at the time fighting the real-estate mogul's legal battles with residents of his properties — until a few months ago.

Rep. Eric Swalwell, a Democrat on the House Intelligence Committee, told CNN on Thursday: "As far as Michael Cohen's credibility, Rudy Giuliani described him as a proven liar, but all of his lies in the past were on behalf of Donald Trump."

Michael Cohen and President Donald Trump. Drew Angerer/Getty Images; Sean Gallup/Getty Images; Jenny Cheng/Business Insider

Trump and his legal team have consistently denied that he had any knowledge of the June 2016 meeting in Trump Tower.

On Friday morning, the president reiterated that claim.

"Sounds to me like someone is trying to make up stories in order to get himself out of an unrelated jam (Taxi cabs maybe?)," Trump tweeted, referring to reports that the FBI is examining Cohen's businesses in New York's taxi industry as part of a separate investigation into the lawyer.

Cohen is the third Trump associate to suggest the president knew more about the 2016 meeting than he's letting on.

Giuliani previously told Business Insider he "would be surprised" if Trump knew about the meeting at the time that it happened. But he also left open the possibility that Trump might have known but later forgot about it.

Giuliani is defending Trump in Robert Mueller's investigation. Shayanne Gal/Business Insider; Alex Wong/Getty; Jonathan Ernst/Reuters

Cohen's lawyer released a tape earlier this week of what is said to be a secretly recorded conversation between Cohen and Trump in September 2016 in which the two men discuss how to buy the rights to a former Playboy model's story of an affair with Trump.

The Thursday reports of Cohen's claims is further evidence that Cohen could flip on Trump in the Russia investigation — which would be a nightmare for Trump's legal team.

"Anyone that takes care of problems and buries the bodies is not someone you want to testify against you," Jeffrey Cramer, a former Justice Department prosecutor, previously told Business Insider's Sonam Sheth.