Oil giant Exxon has clarified a fundraising claim by Donald Trump that he could raise more money than rival Joe Biden.

The US president invoked the company's name at a rally in Arizona, saying all he had to do to raise funds was call Wall Street and oil executives.

He suggested calling Exxon's boss to offer permits in exchange for funds - adding he would never make such a call.

Exxon said on Twitter: "Just so we're all clear, it never happened."

At the rally President Trump gave a scenario of what he could do: "Don't forget, I'm not bad at that stuff anyway, and I'm president.

"So I call some guy, the head of Exxon. I call the head of Exxon. I don't know."

President Trump went on to describe a hypothetical conversation: "How are you doing? How's energy coming? When are you doing the exploration? Oh, you need a couple of permits?"

"When I call the head of Exxon I say, 'You know, I'd love [for you] to send me $25m [£19m] for the campaign.' 'Absolutely sir,'" he added.

"I will hit a home run every single call," Mr Trump said. "I would raise a billion dollars in one day if I wanted to. I don't want to do that."