Speaking during a rally in Oshkosh, Wisconsin, on Monday, the US president said he "should get a redo of four years" - despite the FBI having refuted any claims of spying.

After Franklin D. Roosevelt died while still serving as President, having just started his fourth term in April 1945, Congress passed the Twenty-second Amendment limiting presidents to two terms.

Trump said: "We are going to win four more years. And then, after that, we'll go for another four because they spied on my campaign. We should get a redo of four years."

He had earlier accused his Democrat rival Joe Biden of being a puppet of left-wing extremists who aim to erase America's borders, while in Mankata, Minnesota, on the same day.

"Fascists. They are fascists.

"Some of them, not all of them, but some of them. But they're getting closer and closer," he told supporters.

Trump has also confirmed he will accept the Republican nomination for a second term during a live speech at the White House next week.

Suggestions of the plan had drawn criticism from Democrats and even some fellow Republicans - with some against Trump politicising the historic residence.

Trailing Democrat Joe Biden in opinion polls ahead of the November vote, Trump last week told the New York Post he would "probably" accept the Republican nomination from the White House lawn, calling it "a place that makes me feel good".

The coronavirus pandemic has forced both political parties to overhaul their national conventions.

Rather than an in-person gathering planned for Milwaukee, Wisconsin, Democrats this week have scheduled four nights of virtual speeches and events.

Trump has criticised Democrats for pre-recording some speeches and said he will do a "real speech" next Thursday - the final day of the Republican convention.