Donald Trump has praised late Iraqi leader Saddam Hussein for killing off "terrorists", reprising previous comments in support of dictators and earning a swift rebuke from White House rival Hillary Clinton.

The United States "shouldn't have destabilized" Iraq, now a hotbed for the Islamic State group 13 years after the US-led invasion, said Trump, who showed lukewarm support for the war before it took place and later opposed it.

"He was a bad guy - really bad guy. But you know what he did well? He killed terrorists. He did that so good," Trump told supporters on Tuesday in Raleigh, North Carolina.