Republican presidential candidate Donald Trump's plan to deport 11 million illegal immigrants has been savaged by his rivals in a TV debate.

Two of his rivals for the Republican nomination, John Kasich and Jeb Bush, were very critical, saying his plan was impractical and divisive.

Trump, a billionaire New Yorker, was booed as he tried to counter-attack.

The top eight presidential contenders in the party are squaring off in Milwaukee for the fourth time.

Trump reiterated his calls for a wall to be built at the US-Mexico border and for migrants currently living illegally in the US to be deported.

But he was met with disdain by Kasich, the governor of Ohio.

"Come on, folks, we all know you can't pick them up and ship them back across the border. It's a silly argument. It's not an adult argument."

Bush, the former Florida governor under pressure to revive a flagging campaign, also condemned the plan, saying it would tear families apart.

But New York businessman Trump stood by his proposal by saying illegal immigration hurts the US economy.

Other highlights so far:

-- Trump said the US should look to Israel when it came to building a border wall

-- He also slammed the Trans-Pacific Partnership and China's 'currency manipulation', but seemingly did not realise that China was not party to the trade deal

-- Carson briefly touched on allegations that his memoirs were embellished saying he didn't mind being vetted but had a problem with "being lied about"

-- And he backed the deployment of American troops in Syria

-- Senator Ted Cruz counted the Department of Commerce twice when naming five government agencies he would shut - in a gaffe harkening back to Rick Perry’s 2011 debate meltdown.

-- Rand Paul said income inequality was worst in cities and states run by Democrats

The debate, hosted by Fox Business, began by talking about minimum wage, which several candidates opposed.

Florida Senator Marco Rubio said vocational education was instead a better way to unlock American potential.

"Welders make more money than philosophers. We need more welders and less philosophers."

Neurosurgeon Ben Carson has been neck-and-neck with business mogul Donald Trump leading recent polls.

This debate is the first time fewer than 10 candidates have appeared on stage at once.

The octet on stage each polled at 2.5% or higher in an average of the four most recent polls.

Those who could not muster the minimum were relegated to the so-called "undercard debate", which was held before.

The primary elections, in which the Republican party and the Democratic party pick their presidential nominees, begin in February in the state of Iowa.

And the nation will go to the polls in November to finally pick its new president.