Fox News has apologized to activist Greta Thunberg after a conservative pundit called her a 'mentally ill Swedish child'.

The jibe came after the 16-year-old climate activist gave a highly emotive speech to the UN in New York on Monday.

On Fox News's The Story, conservative writer and The Daily Wire host, Michael Knowles, branded the Swedish teenager 'ill' and said she was 'being exploited by her parents and the international left'.

Just a few hours later another Fox News host took aim at the schoolgirl by comparing her to the kids in the Stephen King horror film, Children of the Corn.

Laura Ingraham asked, 'does anyone else find that chilling?' as a video of Thunberg denouncing world leaders played in the background.

Environmental activist Greta Thunberg addressing the Climate Action Summit in the United Nations General Assembly, at UN headquarters on Monday

Michael Knowles described Greta Thunberg as a 'mentally ill Swedish child' following her speech at the UN

Holy shit



Michael Knowles of Daily Wire just called Greta Thunberg a "mentally ill Swedish child."@ChristopherHahn tore into him pic.twitter.com/Ki0cK6W3Ev — jordan (@JordanUhl) September 23, 2019

Knowles told viewers on Monday night he believed Thunberg was part of 'climate hysteria' that was not about science.

He said: 'The climate hysteria movement is not about science. If it were about science, it would be led by scientists, rather than by politicians and a mentally ill Swedish child who is being exploited by her parents and by the international left.'

Liberal podcast host Chris Hahn, who was also a guest on the show, told Knowles he should be ashamed of his comments.

He said as Knowles began to interrupt: 'Relax, skinny boy. I got this. You're attacking a child. You're a grown man.'

Knowles replied: 'She is mentally ill. She has autism. She has obsessive compulsive disorder, she has selective mutism. She had depression.'

But Fox News apologized for the comments and said the network had 'no plans' for Knowles to appear as a guest in the future. He also writes opinion pieces for FoxNews.com.

Youth Climate activist Greta Thunberg told UN delegates 'you have stolen my dreams and my childhood' as she denounced world leader's inaction on climate change

Michael Knowles described Greta Thunberg as a 'mentally ill Swedish child' following her speech at the UN

A Fox News spokesman told the Hollywood Reporter: 'The comment made by Michael Knowles who was a guest on The Story tonight was disgraceful - we apologize to Greta Thunberg and to our viewers.'

Knowles defended his comments saying the teenager's own mother wrote about her mental health problems in the past.

He tweeted: 'There is nothing shameful about living with mental disorders. Your suggestion to the contrary is not only wrong but deeply offensive.

'Did Greta's mother insult her when she wrote a book describing the child's struggles with mental and developmental disorders?'

Thunberg, who began her campaign by striking from school in Sweden to protest inaction over climate change, previously described her autism as a 'superpower'.

Swedish environmental activist Greta Thunberg watched President Donald Trump intently as he entered the United Nations to speak with reporters in New York

She tweeted in August during her sailing voyage from the UK to New York: 'When haters go after your looks and differences, it means they have nowhere left to go. And then you know you’re winning.

'I have Aspergers and that means I’m sometimes a bit different from the norm. And - given the right circumstances- being different is a superpower.'

Thunberg made international headlines last week as she inspired millions of people across 150 countries to take to the streets for the Global Strike 4 Climate on Friday.

Before the strike, the 16-year-old Swede addressed Congress and criticized President Donald Trump's decision to withdraw the US from the Paris Climate Change Agreement early in his tenure.

Thunberg made a stand against climate change in 2018 when she skipped school to stage a one-person demonstration outside Sweden's parliament in Stockholm.

Her protest ignited a nationwide movement in Sweden thanks to social media and earned Thunberg a Nobel Prize nomination.