Former US Secretary of State John Kerry has responded to President Donald Trump's accusations he had 'illegal meetings with the very hostile Iranian Regime'.

Trump posted a tweet late Thursday saying that the meetings were 'to the detriment of the American people'.

During an appearance on Real Time with Bill Maher Friday, Kerry compared the leader's behavior to that of children.

'He really is the rare combination – he’s got the maturity of an eight-year-old boy with the insecurity of a teenage girl. That’s just who he is,' the former Massachusetts senator told the show's host and audience.

John Kerry called Donald Trump a 'rare combination' – saying 'He’s got the maturity of an eight-year-old boy with the insecurity of a teenage girl' on Friday's Real Time with Bill Maher show

Kerry also said: 'He's the first President who spends more time reading his Twitter likes than his briefing books or the constitution of the United States'

Kerry added that the president needs to spend more time addressing issues that matter.

He mentioned he should be worrying about the plea deal Paul Manafort has struck with Robert Mueller to cooperate with the Russia investigation, saying: 'Soon he'll be saying that's the worst deal that's been made'.

'He's the first President who spends more time reading his Twitter likes than his briefing books or the constitution of the United States,' he said during the interview.

'It's gone from the art of the deal to the art of the squeal,' he said, mentioning the anonymous op-ed that recently appeared in the New York Times where a member of Trump's administration revealed they and other members of the team have vowed to take him down from the inside.

The Vietnam war veteran was on the show to promote his new memoir Every Day Is Extra and urged the audience to see 2017's Netflix film The Vietnam War: A Film by Ken Burns and Lynn Novick, which documents just how early the lying about the reason for the war began.

He compared the lack of truth surrounding that war to current times.

President Donald Trump accused former Secretary of State John Kerry of holding ‘illegal meetings’ with Iran

The Vietnam war veteran was on the show to promote his new memoir Every Day Is Extra

Kerry said: 'We have a president where the truth, the whole truth and nothing but the truth are three different things.'

Maher told Kerry that 2004 was really the beginning of the Fake News that Trump speaks of so much, with a campaign against Kerry who went on to take up a role with former President Barack Obama.

Kerry and Maher spoke about the 'Mean Girls' mentality coming from Trump's office with Kerry ultimately defending his decision to have discussions with Iran since leaving his role.

Under President Barack Obama, Kerry was the lead US negotiator opposite the Iranians in the nuclear talks. He is seen right with Iranian Foreign Minister Javad Zarif at the United Nations headquarters in New York in April 2016

'We've always relied on our democracy getting stronger when people tell the truth,' Kerry continued, as he discussed with Maher that Trump wasn't a fan of the real truth emerging.

Kerry continued by explaining how he and the late John McCain had brought America too far forward for them to regress under Trump's administration.

'John McCain and I together decided we were going to end the war at home and abroad,' he said, adding that he believes all veterans who came back from Vietnam had a very difficult time.

The chat also detailed how they spent 10 years providing families of the deceased with closure by bringing remains back.

Continuing to defend his meetings with Iranian Foreign Minister Javad Zarif for nuclear talks that could change the West's relationship with the Middle East, he said: 'I did absolutely [meet with Iran]. Everybody does. There's absolutely nothing unusual about it.'