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(CNN) —  

Secretary of State Mike Pompeo blasted his predecessor John Kerry for “unseemly and unprecedented” behavior after Kerry said he had met Iranian officials since leaving office.

“What Secretary Kerry has done is unseemly and unprecedented,” Pompeo told reporters at the State Department Friday.

“This is a former secretary of state engaged with the world’s largest state sponsor of terror and according to him, he was talking to them, he was telling them to wait out this administration,’ Pompeo added.

“You can’t find precedent for this in US history and the secretary ought not engage in this kind of behavior,” Pompeo continued, in answer to a question. “It’s beyond inappropriate.”

During interviews to promote his new book, Kerry told an interviewer that he has met with Iranian Foreign Minister Javad Zarif three or four times since leaving office and that their talks touched on the international nuclear agreement, to which Iran, the EU, Russia and China still adhere.

A spokesman for Kerry ridiculed Pompeo’s assertion that Kerry was acting in unprecedented ways, adding that Kerry had briefed Pompeo on his conversations with Iranian officials so he would be aware of Iran’s thinking.

“Secretary Kerry stays in touch with his former counterparts around the world just like every previous Secretary of State, and in a long phone conversation with Secretary Pompeo earlier this year, he went into great detail about what he had learned about the Iranian’s view,” Matt Summers said. “No secrets were kept from this administration.”

“Like America’s closest allies, Kerry believes it is important that the commitments Iran made under the nuclear agreement, which took the world years to negotiate, remain effective,” Summers added. “He was advocating for what was wholly consistent with US policy at the time. There’s nothing unusual, let alone unseemly or inappropriate, about former diplomats meeting with foreign counterparts.”

Kerry, who developed a relationship with Zarif over the course of years-long negotiations for the Iran nuclear deal, has publicly criticized Trump’s decision to walk away from the deal without a replacement.

’Dangerous and ill-advised’

In a Sept. 9 interview with CNN’s Fareed Zakaria, Kerry said the withdrawal was “a very dangerous and ill-advised move that is not based on any broad strategy that is drawing other countries to the table to be supportive of it. Rather, I think it represents a campaign promise made by the President … which has no basis in achieving the goals the President has set out, if there are goals.”

On Thursday, Trump tweeted that Kerry’s meetings were “illegal.”

“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,” Trump tweeted. “He told them to wait out the Trump Administration!”

Kerry responded on Friday, tweeting to Trump that he should be more concerned about the guilty plea of his former campaign manager Paul Manafort and cheekily including a link for Trump to buy his book online.

“Mr. President,” Kerry tweeted, “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.”

In a second tweet Kerry added, “PS - I recorded the audio version, not Omarosa.”

Asked if he thought Kerry’s meeting had been illegal, Pompeo said he would leave the “legal determinations to others.”

He added that, “actively undermining US policy as a former secretary of state is literally unheard of.”

Pompeo conveyed that he had been aware of at least some of Kerry’s meetings, telling reporters that Kerry, along with lead US negotiator Wendy Sherman and former Energy Secretary Ernest Moniz, had been present at an international security conference in Germany he had attended. Describing the former Obama administration officials as “the troika,” Pompeo said, “I am confident that they met with their troika counterparts,” referring to Iranian officials.

And he added, in a reference to Kerry, that “I wasn’t in the meeting but I am reasonably confident he wasn’t there in support of US policy.”

After Donald Trump’s election but before he became president and took office, his transition team was accused of reaching out to the governments of both Israel and Russia to undermine Obama administration policies.

Pompeo singled out Russia for undermining sanctions pressure on North Korea to dismantle its nuclear and ballistic missile programs. “Russia has actively attempted to undermine the UN Security Council Resolutions,” Pompeo said.

Sanctions, the top US diplomat said, “are central to President Trump’s efforts to convince Chairman Kim that full, final, denuclearization of the Korean Peninsula is necessary and that it needs to be done in a way that the world can see that there’s been a strategic change in Chairman Kim’s core understanding of how he will provide a better future for the North Korean people.”

Pompeo also pointed to Russia as he touted Trump’s recent executive order mandating sanctions on anyone interfering in US elections.

“Russia has been particularly aggressive in using its cyber capabilities, disinformation and other covert means to attempt to sow instability in America,” Pompeo said. The “State Department will continue to work closely with other agencies to identify, expose – and expose foreign interference directed against American elections no matter which entity initiated it.”

Critics have said that the executive order lacks teeth, as it is up to the President to decide whether he wants to pursue sanctions or not, and that it is largely redundant. An executive order authorizing sanctions for cyber-related attacks on the US already exists, signed by President Barack Obama.

CNN’s Jennifer Hansler, Elise Labott and Michelle Kosinski contributed reporting.