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.

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 on 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 advise that he should be giving the Iranian government is stop supporting terror groups around the world.”