The Republican candidates for president traded sharp blows over foreign policy and the future of the Supreme Court in an often unruly and chaotic debate.

After Iowa and New Hampshire, the race has now moved to South Carolina before the 20 February primary.

Front-runner Donald Trump repeatedly tangled with Texas Senator Ted Cruz and former Florida Governor Jeb Bush in a series of tense exchanges.

The lively audience repeatedly jeered and booed the candidates.

 Trump and Bush clashed over the war in Iraq and President George Bush's role during the 11 September attacks.

Jeb Bush responded robustly to Trump's attacks, a departure for the former governor who originally sought a "joyful" campaign.

"We should have never been in Iraq," Trump said.

"They lied. They said there were weapons of mass destruction. There were none and they knew that there were none."

 Bush pushed back, defending his brother who will soon join Bush on the campaign trail before the pivotal South Carolina primary.

"I'm sick and tired of Barack Obama blaming my brother for all the problems that he's had and frankly I could care less about the insults Donald Trump gives against me," Bush said.

The death of US Supreme Court Justice Antonin Scalia was addressed early on in the debate with all six candidates saying the next president should choose his replacement.

 Cruz said "we are one justice away" from the court reversing conservative legal gains.

 Trump said President Obama would probably pick a replacement, but stressed that Republicans in the Senate should block him.

"Delay, Delay, Delay," Trump said.

Later, Cruz contended that Trump, who has supported Democrats in the past, would nominate liberal Supreme Court justices if elected president.

"You are the single biggest liar," Trump said to Cruz. "This guy will say anything."

Debate highlights:

--Florida Senator Marco Rubio again clashed with Cruz on immigration reform.  Rubio said Cruz's shifting position on immigration was just another example of his dishonesty.

--Ohio Governor John Kasich continued to push what he sees as a positive campaign message, decrying the sharp attacks of the night.

--Trump said he would work with Congress to penalise companies which move factories and jobs to Mexico.