WASHINGTON - In a prime time address Sunday night, US President Joe Biden said that the political rhetoric in this country has gotten "very heated", and "it's time to cool it down".

Biden made the remarks from the White House Oval Office after Saturday's attempted assassination of former president Donald Trump at a rally in Butler, Pennsylvania, which left one spectator dead and two critically injured.

"There is no place in America for this kind of violence -- for any violence ... We can't allow this violence to be normalized," Biden said.

Noting that the stakes in this election were "enormously high", Biden said "we must never descend into violence" no matter "how strong our convictions" were.

In a brief remark earlier in the day, Biden said that the Federal Bureau of Investigation (FBI) is leading the shooting investigation, which is still "in its early stages".

"We don't yet have any information about the motive of the shooter. We know who he is," said Biden. He also urged Americans not to make any assumption about the motives or affiliations of the shooter.

Biden also noted that he has directed an independent review of the national security at Saturday's Trump rally to assess exactly what happened. The US Secret Service is facing scrutiny after a 20-year-old man, identified by the FBI as Thomas Matthew Crooks, armed with an AR-style rifle was able to get close enough to shoot and injure the former president.

Trump, who was shot in the right ear, arrived in Milwaukee, Wisconsin, on Sunday afternoon, where he is expected to be officially nominated as the Republican presidential candidate later this week.

"Based on yesterday's terrible events, I was going to delay my trip to Wisconsin, and the Republican National Convention, by two days, but have just decided that I cannot allow a 'shooter,' or potential assassin, to force change to scheduling, or anything else. Therefore, I will be leaving for Milwaukee, as scheduled," Trump said in a post on his social media platform Truth Social.

Earlier in the day, Trump urged Americans in another post to stand united and not allow evil to win.

Crooks was a registered Republican but had also given 15 US dollars to a progressive group on Biden's Inauguration Day, more than three years ago, The New York Times reported.

The New York Times said that the shooting came at a time when the United States was so polarized that 47 percent of Americans considered a second civil war likely or very likely in their lifetime.