Republican presidential candidates Donald Trump and Jeb Bush clashed angrily over the Iraq war, the Bush family and Mr Trump's business dealings in a debate ahead of the crucial South Carolina's primary.

Key points: Donald Trump attacks George W Bush for launching Iraq war

Donald Trump attacks George W Bush for launching Iraq war Ted Cruz says Mr Trump will appoint liberals to the Supreme Court

Ted Cruz says Mr Trump will appoint liberals to the Supreme Court Senator Cruz and Marco Rubio battle over illegal immigration

With the Republican candidates increasingly anxious for a good showing in the first primary in the South on February 20, fights between Mr Trump and Mr Bush dominated the two-hour debate.

Mr Trump came under sustained fire from his rivals and there were also finger-pointing exchanges between Mr Trump and Texas senator Ted Cruz.

Senator Cruz and Florida senator Marco Rubio, competing with Mr Bush to emerge as the main anti-Trump alternative, battled over illegal immigration.

Mr Trump, leading polls in South Carolina and in position to take command of the Republican nomination fight if he wins, attacked Mr Bush's brother, former President George W. Bush, for launching the Iraq war in 2003.

"A big, fat mistake," Mr Trump said.

It was the most bitter exchange between them over the course of nine debates ahead of the November 8 presidential election and was a sign of how critical both see a strong showing in South Carolina.

"They lied — they said there were weapons of mass destruction. And there were none," Mr Trump said.

Many in the crowd booed Mr Trump, and the Republican front-runner dismissed them as "lobbyists and special interests" who supported Mr Bush, the former Florida governor.

Mr Bush also criticised Mr Trump for remarks praising Russian President Vladimir Putin, saying Mr Putin is stirring turmoil in Syria by launching air strikes in support of Syrian President Bashir Assad, who Washington would like to leave power.

Jeb Bush said he was "sick and tired" of Mr Trump going after his family. ( REUTERS/Jonathan Ernst )

Bush defends family following Trump attacks

Mr Bush, who will campaign on Monday with his brother, rejected Mr Trump's comments and defended his family, including father former president George HW Bush.

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"I'm sick and tired of him going after my family. My dad is the greatest man alive in my mind," he said.

"While Donald Trump was building a reality TV show, my brother was building a security apparatus to keep us safe. And I'm proud of what he did."

Mr Bush went on the attack against Mr Trump by reminding the audience that the billionaire had criticised his 90-year-old mother, Barbara Bush.

On both Twitter and at a campaign rally last week, Mr Trump mocked Mr Bush for bringing his "mummy" on the campaign trail.

"He had the gall to go after my mother. My mother is the strongest woman I know," Mr Bush said.

Mr Trump responded: "She should be running."

Mr Trump's attacks on the Bush family carried risks for him, since many US military veterans in South Carolina have long supported the family.

Rubio and Cruz renew battle over illegal immigration

Mr Trump also had a heated exchange with Senator Cruz when the senator said Mr Trump would appoint liberals to the Supreme Court.

"You are the biggest liar," Mr Trump said sharply in reply.

As they tried to talk over each other, Senator Cruz chided Mr Trump by saying: "Adults don't interrupt."

Mr Cruz and Mr Rubio renewed their battle over who is the toughest on illegal immigration with Mr Cruz insisting that Mr Rubio — as part of a Gang of Eight senators who sought a compromise on legislation in 2013 — was for "amnesty" but now is against it for political purposes.

He insisted that Mr Rubio had said in Spanish on Univision that he would not rescind an executive order signed by President Barack Obama in support of the children of illegal immigrants.

Mr Rubio shot back: "I don't know how he knows what I said on Univision because he doesn't speak Spanish."

As the crowd roared, Mr Rubio said Senator Cruz was "telling lies".

Reuters