Fox News has fired back at Donald Trump for his latest remarks about anchor Megyn Kelly, who has been the target of repeated attacks by the GOP frontrunner for the presidential nomination.

Fox News condemned Trump’s “vitriolic attacks” and “sick obsession” with Kelly, one of the network’s most prominent anchors and host of the 9 p.m. nightly news talk show “The Kelly File.” The no-holds-barred statement came after Trump sent a tweet once again criticizing Kelly and calling for viewers to stop watching her program.

“Donald Trump’s vitriolic attacks against Megyn Kelly and his extreme, sick obsession with her is beneath the dignity of a presidential candidate who wants to occupy the highest office in the land,” Fox News said 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. 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 her to be repeatedly abused just for doing her job.”

Sources said Fox News insiders were unclear what prompted Trump’s Twitter slam against Kelly on Friday. But Trump has unleashed a steady stream of tweets this week attacking Kelly for what he characterized as unfair coverage of his campaign.

“Everybody should boycott the Megyn Kelly show,” Trump tweeted. “She is sick and the most overrated person on TV.”

Trump’s campaign fired back at Fox News’ comments on Friday evening: “Megyn Kelly is a highly overrated reporter and anchor that constantly disparages Mr. Trump with negative and inaccurate reports. Despite the fact he wants nothing to do with her and will not appear on her show due to her extremely biased reporting, much of the program is about him anyway on a nightly basis. In fact, FOX News has begged Mr. Trump to do a prime time special to be broadcast on the FOX Network, not cable, with Megyn Kelly. He has turned them down.”

They added, “Likewise, they recently called for an unsolicited and unplanned debate with Kelly as a featured moderator. He turned them down and they immediately canceled. On the FOX debate that Mr. Trump did not do, they received by far their lowest rating. 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.”

Fox News officials are growing increasingly concerned for Kelly’s safety as Trump continues to target the anchor for comments that would be unthinkable coming from virtually any other contender for the White House.

The outbreaks of violence at Trump campaign events in recent weeks have spurred much commentary about a rising tide of anger among Trump supporters, fueled by the candidate’s stances on immigration, Muslims entering the U.S. and his general savaging of the existing political establishment. The unpredictability of Trump’s comments on Kelly has also become unnerving to Fox News staffers.

On Thursday, Trump derided the Fox News anchor for “always complaining” about his campaign, hours after calling her “crazy” and again, “highly overrated.”

Trump and Kelly first tangled last August when she was one of the moderators of a Republican candidates debate hosted by Fox News. Trump slammed her for being unfair to him after tough questioning about his past derogatory statements about women.

Since then, Trump and Fox News have had a roller coaster relationship, with both sides trading barbs at times and Trump engaging in an on-again, off-again boycott of the channel. The situation is all the more remarkable given Fox News’ long-standing profile as a friendly outlet to conservatives in its opinion programming with hosts such as Bill O’Reilly and Sean Hannity.

On March 3, Trump faced off against Kelly in another Fox News hosted debate, without any fireworks. But earlier this week Trump said he would not take part in a Fox News debate that had been scheduled for March 22 in Salt Lake City, citing a prior speaking engagement. Trump’s decision prompted Fox News to table the debate.

Kelly’s star has been rising at Fox during the past few years. She is set to host a primetime Barbara Walters-style interview special on Fox in May. And she will be feted as one of Variety‘s honorees at the April 8 Power of Women New York luncheon highlighting prominent industry women and their charitable endeavors.