John Kerry, a former secretary of state, appeared  Friday on “Real Time with Bill Maher” and almost immediately attacked President Donald Trump. The two have been feuding on Twitter this week after revelations that Kerry has continued meeting with Iran to discuss a potential deal with that country.

In the Maher interview, Kerry said, “He is the first president that I know of who spends more time reading his Twitter ‘likes’ than his briefing books or the Constitution of the United States.”

He also asserted to Maher that Trump had “the maturity of an eight-year-old boy with the insecurity of a teenage girl.”

The former presidential candidate served as then-President Barack Obama’s secretary of state and was one of the chief architects of the Joint Comprehensive Plan of Action (JCPOA), commonly referred to as the Iran Deal, from which President Trump withdrew earlier this year.

The president cited issues with compliance and also disapproval of the allegedly lax standards for inspection.

The deal had not been approved by the Senate as a treaty due to Republican objections; Trump was able to withdraw of his own accord, without requiring an act of Congress.

Related: ‘It’s in Our Best Interests’ to Get Out of Flawed Deal, Bolton Argues

It appears now that Kerry has met with Iranian officials in an attempt to renegotiate or otherwise manage diplomacy with the United States.

As President Trump tweeted on Thursday, “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.”

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The law he’s referring to is the Logan Act, which was passed in 1799. That makes it illegal to attempt to conduct diplomacy with foreign governments without the government’s permission.

However, only two people have ever been charged with violations of the Logan Act — and neither of them was convicted.

Related: John Kerry Blasted for Shadow Diplomacy with Iran

In other interviews, Kerry was asked whether he had told the Iranians they should just wait until a new, presumably Democratic president was elected, rather than assuming that Trump’s decision to pull out of the JCPOA would stand. He did not answer the question directly.

Kerry, for his part, responded by saying that Trump should care more about special counsel Robert Mueller’s investigation into possible Russian collusion than what Kerry is doing.

“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, Every Day Is Extra,” he tweeted.

In the interview with Maher, Kerry appeared to dismiss concerns over his meetings with the Iranian, comparing them to the actions of Kissinger: “Well, Henry Kissinger, for 40 years, has been traveling to Russia, traveling to China — talks with the leaders. There’s absolutely nothing unusual about it.”

Check out these reactions to Kerry’s remarks — then watch the interview with Maher:

https://twitter.com/FoxNews/status/1041202520989679616