Ted Cruz in Iowa
US Senator Ted Cruz has been forced to take action to clean up his campaign's image Reuters

US Republican presidential candidate Ted Cruz has addressed his growing credibility gap by sacking his communications director for issuing a false video that accused rival Marco Rubio of disparaging the Bible.

It was an awkward move a day before the Nevada caucus.

The sacking is the latest indication that the Texas senator is concerned about the mounting attacks against him, first launched by front-runner Donald Trump, that he plays fast and loose with the truth and is a master of dirty tricks.

Communications director Rick Tyler was forced to apologize after posting a story on Facebook that falsely claimed Rubio told Cruz's father that the Bible didn't "have many answers" in it.

The story included a video with inaccurate subtitles. In fact, Rubio was praising the Bible and said the book has "all the answers."

"I've spent this morning investigating what happened, and ... I asked for Rick Tyler's resignation," Cruz told reporters at a news conference in Las Vegas. "I have made clear in this campaign that we will conduct this campaign with the very highest standards of integrity."

Cruz called Tyler a "good man," but said the Facebook story was "a grave error in judgment."

Rubio's campaign immediately stepped up to blast Cruz's credibility.

"Rick is a really good spokesman who had the unenviable task of working for a candidate willing to do or say anything to get elected," Rubio communications director Alex Conant told the Daily Beast. "There is a culture in the Cruz campaign, from top to bottom, that no lie is too big and no trick too dirty."

Rubio's campaign had earlier complained about a doctored photograph used by the Cruz campaign that suggested Rubio had shaken hands with President Obama over a controversial trade pact.

Trump, too, took the opportunity to say "I told you so." He first called Cruz a liar in the run-up to the Iowa caucuses when Cruz accused him of supporting the Obamacare health insurance program.

At a press conference before the South Carolina primary he called his rival a liar six times in 90 seconds, noting: "He's the single biggest liar I've ever seen."