US President Donald Trump and Secretary of State Mike Pompeo have accused Obama-era official John Kerry of "actively undermining" US policy on Iran after he admitted meeting Iran's foreign minister "three or four times" since leaving office.
"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 said in Twitter post on Friday.
"He told them to wait out the Trump Administration!" he said, ending his Tweet with the word "BAD!"
Meanwhile, Pompeo told a news conference that Kerry's meetings with Mohammad Javad Zarif were "inconsistent" with US foreign policy and "beyond inappropriate".
"You can't find precedent for this in US history, and Secretary Kerry ought not to engage in that kind of behaviour," an agitated Pompeo told reporters at the State Department.
"It's inconsistent with what foreign policy of the United States is as directed by this president, and it is beyond inappropriate for him to be engaged."
Kerry, who negotiated the 2015 Iran nuclear deal which Trump scrapped this year, said during a tour to promote his new book "Every Day is Extra" that he had met Zarif "three or four times" since Trump had entered the White House.
A spokesperson for Kerry said in a statement to The Hill on Friday: "Kerry stays in touch with his former counterparts around the world just like every previous secretary of state.
"There's nothing unusual, let alone unseemly or inappropriate, about former diplomats meeting with foreign counterparts ... What is unseemly and unprecedented is for the podium of the State Department to be hijacked for political theatrics," the spokesperson added.
Meanwhile, Kerry took to Twitter to respond to Trump, saying: "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."
Trump supporters immediately leapt on Kerry's actions as evidence of "treason," with some calling for him to go to prison.
A spokesperson for Kerry said there was nothing improper about his conduct.
Tensions between Iran and the US have risen after Washington pulled out of the 2015 Iran nuclear deal.
Under the 2015 deal, the Iranian government agreed to cut down its uranium stockpile and scale back its enrichment programme far below the level required to build a nuclear weapon.
Tehran also agreed in perpetuity to notify United Nations inspectors if and when it builds a new nuclear facility.
In exchange, UN-approved sanctions were lifted in January 2016, and Tehran was allowed to resume trading oil and gas on the international market. A total of $100bn in frozen Iranian assets was also released.
Following the US pullout from the nuclear agreement in May, Iran said it was interested in keeping the deal alive, but only if the remaining powers could guarantee it would not face economic isolation under Washington's sanctions.
Europe and other signatories of the deal oppose the US' decision to leave the agreement and have vowed to find ways of maintaining trade ties with Iran.