Washington (AFP) - US President-elect Donald Trump said Wednesday that advice from a Marine general renowned for his frank talk had convinced him to think again about authorizing the torture of detainees.

The Republican tycoon had previously made bloodcurdling pledges to restore waterboarding and "far, far worse" in US detention centers, scorning President Barack Obama's ban on the practice.

But last weekend Trump sat down with James Mattis, a retired general respected across Washington and the US military for his no-nonsense approach, to discuss naming him defense secretary.

On Tuesday, in an interview with the New York Times, Trump said he'd been surprised to discover that such an uncompromising commander was opposed to allowing US personnel to torture suspects.

"He said: 'I've never found it to be useful'," he said, adding that Mattis advised building a rapport with detainees: "Give me a pack of cigarettes and a couple of beers and I'll do better."

"I was very impressed by that answer," Trump told the Times, in a marked U-turn from his campaign mantra: "Torture works, OK? Believe me, it works."

Trump's rhetoric was cheered to the rafters at campaign rallies, but on Tuesday he admitted that torture is "not going to make the kind of a difference that a lot of people are thinking."

Mattis is reportedly interested in becoming Trump's defense secretary, and Trump said he is being "seriously, seriously considered" despite his opposition to illegal interrogation methods.