Republican presidential candidate Donald Trump has softened his stance on torture, saying he would not order the US military to break international laws on how to treat terrorism suspects, the Wall Street Journal reports.

"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 in a statement to the newspaper on Friday.

"I will not order a military officer to disobey the law. It is clear that as president I will be bound by laws just like all Americans and I will meet those responsibilities."

In a Republican presidential debate on Thursday night, Trump indicated he might order the US military to break the law on interrogation tactics including waterboarding.

Related reading Trump, Clinton hope for nomination leads

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

Critics say it is torture, and Democratic President Barack Obama banned the use of the method days after taking office in 2009.

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? 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?

"We should go for waterboarding and we should go tougher than waterboarding," Trump said on the Fox News debate.