US Republican presidential candidate Donald Trump retreated from his promise that as an elected president he would order the military to waterboard those threatening the United States and have their families killed.

The billionaire businessman said in a TV debate on Thursday the military should "go tougher than waterboarding", and he defended his comments in December suggesting when it comes to "terrorists", the US must "take out their families".

However, Trump's campaign issued a statement on Friday quoting him as saying he would not order the military to take actions contrary to international or US law.

The use of torture and the killing of civilians are barred by the Geneva Conventions, to which the US is a signatory.

"I do, however, understand that the United States is bound by laws and treaties and I will not order our military or other officials to violate those laws and will seek their advice on such matters," Trump said. "It is clear that as president I will be bound by laws, just like all Americans, and I will meet those responsibilities."

READ MORE: What would Donald Trump mean for the Middle East

Waterboarding is the practice of pouring water over someone's face to mimic drowning as an interrogation tactic. Critics call it torture.

Congress outlawed waterboarding and other "enhanced interrogation techniques" after the administration of George W Bush carried out such acts against suspected al-Qaeda fighters following the September 11 attacks.

Backlash against Trump

Trump's comments drew attention after more than 100 Republican defence and national security figures, including former senior Pentagon officials, issued a statement blasting his foreign policy positions and calling his embrace of the expansive use of torture "inexcusable".

READ MORE: Super Tuesday: What's behind the rise of Donald Trump?

Asked during the debate what he would do if the US military refused to carry out such orders, Trump said, "They won't refuse. They're not going to refuse me. Believe me.

"Can you imagine these people, these animals over in the Middle East, that chop off heads, sitting around talking and seeing that we're having a hard problem with waterboarding?"

He added: "When you get these terrorists, you have to take out their families. They care about their lives, don't kid yourself. When they say they don't care about their lives, you have to take out their families."