Fox News on Friday issued a harsh condemnation to Donald Trump in defence of one of the network's most popular reporters, calling the Republican front-runner's "obsession" unfit for a presidential candidate.

"Donald Trump's vitriolic attacks against Megyn Kelly and his extreme, sick obsession with her is beneath the dignity of a presidential candidate," the conservative cable news network wrote in a statement.

"Megyn is an exemplary journalist and one of the leading anchors in America - we're extremely proud of her phenomenal work and continue to fully support her throughout every day of Trump's endless barrage of crude and sexist verbal assaults."

The statement from Fox, which is part of the Rupert Murdoch-controlled Twenty-First Century Fox Inc, comes in response to a recent onslaught of negative comments from Trump to Kelly, re-sparking a months-long feud between the two.

Trump irked by August 2015 debate question

The quarrel began in August when, during a Fox News-hosted Republican debate, Trump blasted Kelly over questions she posed to him regarding his history with women, including his references to some women he didn't like as "dogs" and "fat pigs."

Trump upped the ante shortly thereafter, saying that Kelly had become so incensed, "you could see there was blood coming out of her eyes. Blood coming out of her - wherever," which many viewed as Trump insinuating that Kelly was menstruating.

The Trump campaign later issued a statement saying that by "her wherever," Trump meant Kelly's nose.

Still, the damage had been done, and the relationship between the billionaire businessman and the longtime Fox News anchor has remained strained.

Beginning Tuesday, Trump took to Twitter to blast Kelly, tweeting and retweeting a series of messages referring to her as "Crazy Megyn," and calling for supporters to boycott her weeknight news program, The Kelly File.

"Everybody should boycott the @megynkelly show. Never worth watching. Always a hit on Trump! She is sick, & the most overrated person on tv," Trump tweeted on Friday.

Fox News responded by saying: "As the mother of three young children, with a successful law career and the second-highest rated show in cable news, it's especially deplorable for (Kelly)to be repeatedly abused just for doing her job."

The Trump campaign stood by the negative tweets, calling Kelly "highly overrated" in a statement, and accusing her of biased reporting.

"Unlike Megyn Kelly, who resorts to putting out statements via Fox News, Mr. Trump will continue to defend himself against the inordinate amount of unfair and inaccurate coverage he receives on her second-rate show each night," the campaign said.