Bourse and Bazaar | AFP: President Donald Trump lashed out at former secretary of state John Kerry for his meetings with Iran's foreign minister after the Obama-appointee had left 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 on Twitter late Thursday.

"He told them to wait out the Trump Administration!" he said, ending his Tweet with the word "BAD!"

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 Iranian Foreign Minister Mohammad Javad Zarif "three or four times" since he left office and Trump had entered the White House.

Without commenting on the legality of such meetings, the current US Secretary of State, Mike Pompeo, accused his predecessor of "actively undermining US policy."

"What secretary Kerry has done is unseemly and unprecedented," said Pompeo, who is seen as an anti-Iran hawk.

"This is a former secretary of state engaged with the world's largest State sponsor of terror and according to him... he was telling them to wait out this administration."

A spokesman for Kerry said there was nothing improper about his conduct.

"Secretary Kerry stays in touch with his former counterparts around the world just like every previous Secretary of State," the spokesman said in a statement.

"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."

"No secrets were kept from this administration," the statement said, accusing Kerry's successor of "political theatrics."

Straight Talk

Asked by conservative radio host Hugh Hewitt on Wednesday if he had offered Zarif advice on how to deal with Trump's decision to withdraw from the pact, he replied: "No, that's not my job.

"What I have done is tried to elicit from him what Iran might be willing to do in order to change the dynamic in the Middle East for the better.

"I've been very blunt to Foreign Minister Zarif, and told him look, you guys need to recognize that the world does not appreciate what's happening with missiles, what's happening with Hezbollah, what's happening with Yemen," he added, echoing the current administration's denunciation of Tehran's "malign" influence.

Conservative commentators immediately leapt on the act as evidence of "treason," with some calling for Kerry to go to prison.

Asked by a Republican lawmaker during a congressional hearing about the so-called shadow diplomacy, Manisha Singh, an assistant secretary of state, said Thursday: "It's unfortunate if people from a past administration would try to compromise the progress we're trying to make in this administration."

State Department spokeswoman Heather Nauert added: "I've seen him brag about the meetings that he has had with the Iranian government and Iranian government officials. I've also seen reports that he is apparently providing, according to reports, advice to the Iranian government.

"The best advice that he should be giving the Iranian government is stop supporting terror groups around the world."