A war of words has erupted between two Fox News hosts after Tucker Carlson mocked his colleague Shepard Smith over comments about President Donald Trump and his Ukraine call.

Carlson targeted Smith during his show on Wednesday night after the latter had defended a Fox News analyst earlier in the day.

The ongoing saga was ignited on Tuesday when Smith had Fox News contributor and former judge Andrew Napolitano on his show to talk about the Trump-Ukraine call.

Judge Napolitano told Smith that it would constitute a crime if Trump had asked Ukrainian President Volodymyr Zelenskiy to investigate presidential candidate Joe Biden to uncover dirt on him.

Tucker Carlson (left) mocked his Fox News colleague Shepard Smith during his show on Wednesday night when he was interviewing pro-Trump lawyer Joe diGenova (right)

'It is a crime for the president to solicit aid for his campaign from a foreign government,' Napolitano said during Smith's show.

Later that night on Carlson's show, pro-Trump lawyer Joe diGenova slammed the judge as a 'fool' and said his opinions were 'foolish'.

It sparked backlash from Smith who flagged the comments on his show on Wednesday.

Smith said diGenova was a 'partisan guest' and that his comments went unchallenged by prime time host Carlson.

'Attacking our colleague who's here to offer legal assessments on our air in our work home is repugnant,' Smith told his viewers.

In response, Carlson invited diGenova back on his show Wednesday night to mock Smith.

'Apparently our daytime host who hosted Judge Napolitano was watching last night and was outraged by what you said and, quite ironically, called you partisan,' Carlson said.

The saga ignited when Smith (above) defended a Fox News analyst that one of Carlson's prior guests - pro-Trump lawyer Joe diGenova - had slammed as a 'fool'. Smith slammed the comments on his show on Wednesday, saying the attack on a Fox News contributor was 'repugnant'

Smith had Fox News contributor and former judge Andrew Napolitano on his show regarding the Trump-Ukraine call on Tuesday. Judge Napolitano said it would constitute a crime if Trump had asked the Ukraine to investigate presidential candidate Joe Biden to uncover dirt on him

After re-playing Smith's comments from earlier in the day, Carlson told his guest: 'Repugnant. Not clear if that was you or me, but someone was repugnant.'

'Now, unlike maybe some dayside hosts, I'm not very partisan,' Carlson said. 'Is it a crime or not? Given everything that's happened in the last 24 hours. I want to throw it to you again. Was it a crime or not?'

DiGenova reiterated his belief again that it doesn't constitute a crime.

'It doesn't seem honest to me when a host, any host on any channel, including this one, pretends that the answer is obvious,' Carlson said. 'That's not news, is it? That's opinion.

'Why do we find our selves in a situation where people aren't willing to admit that their passions are guiding their news coverage?

'Wouldn't it be better if we just said out loud you know this is what I think? For example you will never hear me criticize Rachel Maddow. I never agree with anything she says. But she is straightforward, it's her opinion. Why wouldn't it be better if we were all that transparent about what's driving our shows?

'It makes people cynical when you dress up news coverage, when you dress up partisanship as news coverage and pretend that your angry political opinions are news, you know, people tune out.'