DES MOINES, United States (AFP) - White House hopeful Donald Trump will not participate in Thursday's (Jan 28) Republican debate, bowing out of a nationally televised showdown just four days before Iowa kicks off the presidential nomination process, US media reported.

Mr Trump, the front runner for the Republican nomination, publicly toyed with the idea of pulling out of the debate during a campaign event on Tuesday (Jan 26) in Iowa, where he said "probably I won't be doing the debate".

An hour later, his campaign manager Corey Lewandowski told The Washington Post that he would "definitely not" participate.

Mr Trump's decision to drop out only heightens his feud with Fox News, the host of Thursday's debate in Iowa and whose moderator Megyn Kelly has been accused by Mr Trump of bias against him.

"I don't think she can treat me fairly. And I'm not a big fan of hers," he told CNN earlier on Tuesday.

Ms Kelly moderated the first debate of the campaign cycle in August when she was tough on Mr Trump.

Afterwards, the billionaire suggested Ms Kelly had treated him unfairly because she was menstruating.

Fox News chairman Roger Ailes, in what looked increasingly like a game of high-powered chicken, gave his moderator his support.

"Megyn Kelly is an excellent journalist and the entire network stands behind her," Mr Ailes told the Post.

"She will absolutely be on the debate stage on Thursday night."

Fox intensified the stand-off by mocking Mr Trump, who had asked his five million Twitter followers to weigh in on whether or not he should attend the debate.

"A nefarious source tells us that Trump has his own secret plan to replace the Cabinet with his Twitter followers to see if he should even go to those meetings" with the leaders of Iran and Russia, Fox News said in a tongue-in-cheek statement.

Mr Trump's absence will leave a huge hole on the Des Moines stage, where fellow Republicans including Senator Ted Cruz, Senator Marco Rubio, retired neurosurgeon Ben Carson and former Florida governor Jeb Bush will be making their final pitch to Iowa voters.

Cruz spokesman Rick Tyler blasted Mr Trump's decision as "erratic" and foolish behaviour from a presidential hopeful, especially considering a large segment of voters say they have not finalised their support for a candidate.

Bailing out on a debate "because he has a problem with the host is just childish", he told CNN.