Former Secretary of State John Kerry John Forbes KerryThe Memo: Warning signs flash for Trump on debates Divided country, divided church TV ads favored Biden 2-1 in past month MORE fired back at President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE on Friday after the president slammed Kerry's continued meetings with foreign diplomats from Iran and other countries, calling them "illegal."

“There’s absolutely nothing unusual about it. The conversation I think he really ought to be worried about is Paul Manafort Paul John ManafortOur Constitution is under attack by Attorney General William Barr Bannon trial date set in alleged border wall scam Conspicuous by their absence from the Republican Convention MORE with Mueller,” Kerry said on HBO’s “Real Time with Bill Maher,” referring to Trump's former campaign chairman who "flipped" on Friday to cooperate with special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE.

“It really is giant, and soon you’ll be hearing him say that is the worst that’s ever been made. When you think about it, it’s really gone from the Art of the Deal to the art of the squeal,” he added.

Mr. President, you should be more worried about Paul Manafort meeting with Robert Mueller than me meeting with Iran's FM. But if you want to learn something about the nuclear agreement that made the world safer, buy my new book, Every Day Is Extra: https://t.co/DKjc33Kvvu https://t.co/cesltkt0zW — John Kerry (@JohnKerry) September 14, 2018

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Manafort agreed to a plea deal with Mueller's team Friday on two federal charges. The deal involves full cooperation with Mueller in his probe into possible collusion between the Trump campaign and Russian meddling in the 2016 election as well as looking at potential obstruction of justice.

Manafort was among the Trump campaign officials who attended the June 2016 meeting in Trump Tower with Donald Trump Jr. Don John Trump'Tiger King' star Joe Exotic requests pardon from Trump: 'Be my hero please' Zaid Jilani discusses Trump's move to cancel racial sensitivity training at federal agencies Trump International Hotel in Vancouver closes permanently MORE and a Kremlin-linked lawyer who had promised dirt on Hillary Clinton Hillary Diane Rodham ClintonDemocratic groups using Bloomberg money to launch M in Spanish language ads in Florida The Hill's Campaign Report: Presidential polls tighten weeks out from Election Day More than 50 Latino faith leaders endorse Biden MORE.

Kerry's comments Friday are the latest in a feud between the pair this week over Kerry’s maintained contacts with foreign dignitaries after he left the State Department when Trump took office in early 2017.

“John Kerry had illegal meetings with the very hostile Iranian Regime, which can only serve to undercut our great work to the detriment of the American people. He told them to wait out the Trump Administration! Was he registered under the Foreign Agents Registration Act? BAD!” Trump tweeted on Thursday.

John Kerry had illegal meetings with the very hostile Iranian Regime, which can only serve to undercut our great work to the detriment of the American people. He told them to wait out the Trump Administration! Was he registered under the Foreign Agents Registration Act? BAD! — Donald J. Trump (@realDonaldTrump) September 13, 2018

Secretary of State Mike Pompeo Michael (Mike) Richard PompeoTreasury sanctions individuals, groups tied to Russian malign influence activities Navalny released from hospital after suspected poisoning Overnight Defense: Pentagon redirects pandemic funding to defense contractors | US planning for full Afghanistan withdrawal by May | Anti-Trump GOP group puts ads in military papers MORE also called the meetings “unseemly and unprecedented” on Friday.

Kerry played a central role during the Obama administration in crafting the nuclear deal Iran signed with the U.S. and five other world powers in 2015, a deal Trump pulled the U.S. out of in May.

The former secretary of State has denied any wrongdoing regarding the meetings with Iran and other countries.

“We have conversations with people about the state of affairs in the world in order to understand them. We don’t negotiate, we’re not involved in interfering with policy,” Kerry said on Fox News earlier this week.

“It’s not Iran deal-specific, it’s with respect to trade, it’s with respect to NATO,” he added.