With just five days left before the South Carolina primary, Republican frontrunner Donald Trump has increased his attacks on rival Ted Cruz and has even threatened to sue him over his eligibility to serve as president. Trump has accused the Texas senator of spreading falsehoods about the real estate mogul and has called on him to apologise.

"One of the ways I can fight back is to bring a lawsuit against him relative to the fact that he was born in Canada and therefore cannot be President," Trump said in a statement. "If he doesn't take down his false ads and retract his lies, I will do so immediately. Additionally, the RNC should intervene and if they don't they are in default of their pledge to me."

According to CNN, the winner of the New Hampshire later held a press conference in which he said he "never ever met a person that lies more than Ted Cruz." In a later interview with CNN's Jim Sciutto, Trump added: "Guys like Ted Cruz — it's all talk and no action ... He's a very bad guy. He's a very nasty guy. He's got no endorsements from any senators who he works with. He'll never be able to unify anything."

Trump did not stop there. He continued: "He can stand in the middle of the Senate floor and let everybody laugh at him, which is what happened last time, as you know — he got nothing out of it, zero — but, you know, that's not the kind of person who you're going to need to turn the country around."

For his part, Cruz released anti-Trump adverts criticising the candidate's past comments on Hillary Clinton and Planned Parenthood. He took to Twitter to share his adverts, adding: "You cannot simply scream 'liar' when someone points out your actual positions".

According to Real Clear Politics, Trump is dominating the field in the next primary state with double-digit leads in all recent polls. Cruz is following behind in second, with Florida Senator Marco Rubio trailing him in an average of two points. The South Carolina Republican primary is set for 20 February, followed by the Nevada GOP caucus on 27 February.