Mike Pompeo accused predecessor John Kerry of "actively undermining" US foreign policy for telling Iranian officials to "wait out" President Donald Trump's time in office.
On Friday, the US Secretary of State criticised Mr Kerry for "unseemly and unprecedented" behaviour while meeting Iranian officials.
Mr Kerry acknowledged this week that he had met Iranian Foreign Minister Mohammad Javad Zarif “three or four times” since 2016.
The Boston Globe reported that in these meetings Mr Kerry advised the Iranians to "wait out" Mr Trump's time in office, and in an interview with Fox News this week, Mr Kerry did not deny nor did he confirm that assertion.
“I think everybody in the world is talking about waiting out President Trump,” Mr Kerry said.
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"This is a former secretary of state engaged with the world's largest state sponsor of terrorism," Mr Pompeo said of Mr Kerry's meetings, adding that last week Iran directed its militias in Iraq to fire rockets at the US consulate in Basra. He also said that he saw Mr Kerry in Munich earlier this year, along with former US energy secretary Ernest Moniz and former US negotiator Wendy Sherman, where he believed they met Iranian officials.
Mr Pompeo criticised Iran for continuing to arm the Houthis in Yemen, and plotting assassinations in Europe. He anticipated full compliance from European states once US oil sanctions on Iran are in place on November 4.
Mr Pompeo did not say that Mr Kerry had broken the law but Mr Trump accused Mr Kerry of doing so on Thursday, tweeting: "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!"
Mr Kerry retorted that the president ought to be more concerned that his former campaign manager, after pleading guilty to money laundering and obstruction of justice charges, is cooperating with Special Counsel Robert Mueller's investigation into Russian interference in the 2016 election.