The son of US Republican presidential nominee Donald Trump has sought to reassure those who might see his father as dangerous.

Donald Trump Jr was speaking on the eve of a debate between his father and Democratic Party rival Hillary Clinton about undecided voters.

He said: "My father's going to have the media, everyone, both sides - Republican, Democrats - watching every move that he makes.

"For those who say: 'Oh he's so dangerous', there is nothing dangerous about it.

"He is going to have the greatest system of checks and balances in place watching him.

"If Hillary Clinton gets in, the media is going to give her a pass as they have with every scandal she's had for the last 30 years, everything that's going to go unchecked.

"The liberals will push that agenda and the Republicans have shown that they're, generally speaking, too weak to actually take them on.

"So the safe candidate is the guy who is going to have the scrutiny of the world watching him and I think that should go a long way with people who are undecided who may not be thrilled with the other choice."

He was speaking to Sky News as the candidates prepare to go head-to-head in a debate at New York's Hofstra University on Monday night local time, with just six weeks to go until polling day.

According to an Associated Press-GFK poll, more than 85% of voters say they will not change their mind about who to vote for, while 13% say they are undecided.