Republican presidential candidate Donald Trump has defended his protectionist trade policies and other fiscal proposals, dismissing suggestions that they could drive up the federal debt, antagonise allies and harm the US economy.

"We're all about growth. We're going to bring companies back to our country," the former US president said in a sometimes-tense interview at the Economic Club of Chicago.

The interviewer, John Micklethwait, editor-in-chief of Bloomberg News, cited projections by budget analysts that Trump's plans would add $US7.5 trillion to the federal debt through the year 2035, more than twice that of policies favoured by Trump's Democratic opponent in the November 5 election, Vice President Kamala Harris.

Trump maintained that his trade policies - which call for pricey tariffs on goods not only from rivals such as China but allies such as the European Union - would revitalise American manufacturing and yield enough revenue to ease concerns about ballooning the deficit.

(Trump's interview with Bloomberg News Editor-in-Chief John Micklethwait was often tense. (AP PHOTO)

Some trade experts have argued those tariffs could damage the US economy, jeopardise jobs and drive up consumer prices.

"All you have to do is build your plants in the United States, and you won't have any tariffs," Trump said. "I agree it's going to have a massive effect, a positive effect, not a negative."

Trump reiterated that he would levy a high tariff on vehicles assembled in and imported from Mexico - as high as 200 per cent. And he said he would impose duties on imported cars from countries such as Germany in order to force foreign companies to manufacture their products in the US.

When Trump was told that his efforts might annoy allies the US needs to compete against China, he responded by saying, "Our allies have taken advantage of us more than our enemies."

Trump also refused to say whether he's spoken with Russian President Vladimir Putin since leaving office, as reported in journalist Bob Woodward's latest book.

Woodward reports in his book War that Trump has had as many as seven private phone calls with Putin since leaving the White House and secretly sent the Russian president COVID-19 test machines during the height of the pandemic.

A Trump campaign spokesperson previously denied the report. During Tuesday's interview, Micklethwait posed the question to Trump directly: "Can you say yes or no whether you have talked to Vladimir Putin since you stopped being president?

(A new book claims Trump had up to seven private calls with Vladimir Putin since leaving office. (EPA PHOTO)

"I don't comment on that," Trump responded. "But I will tell you that if I did, it's a smart thing. If I'm friendly with people, if I can have a relationship with people, that's a good thing and not a bad thing in terms of a country."

Trump spokesperson Steven Cheung called the book's reporting as false. Kremlin spokesperson Dmitry Peskov said the reporting about the calls was "not true."

Meanwhile Kamala Harris defended her record as a prosecutor, pledged to decriminalise marijuana and to push for police reform, as she aimed to shore up support among Black men in an interview with radio host Charlamagne tha God in Detroit.

One of his first questions was asking Harris to address a rumour - that she disproportionately locked up Black men over her more than a dozen years as San Francisco's district attorney.

"It's just simply not true," Harris said, adding she was described as "one of the most progressive prosecutors" on marijuana cases.

She said as president she would work to decriminalise marijuana because she knew how the laws have hurt certain populations, especially Black men.

(Kamala Harris during an interview with Charlamagne Tha God in Detroit. (AP PHOTO)

Harris said one of the biggest challenges she faces is misinformation from the Trump team aimed at Black voters.

"They are trying to scare people away because they know that otherwise they have nothing to run on," she said. 

Asked about how she would curb police brutality and murders of Black men, Harris said she would work to pass the George Floyd Policing Act, which stalled in Congress in 2021.

By James Oliphant in Chicago

with AP