US President Joe Biden on Sunday appealed for the country to “unite as one nation” after Saturday’s apparent attempted assassination of former president Donald Trump, and said he was ordering an independent security review of the lead-up to the attack.

Mr Biden was speaking from the White House after receiving a briefing on the investigation in the Situation Room.

He said he has directed the probe to be “thorough and swift”, and asked the country not to “make assumptions” about the perpetrator’s motives or affiliations.

“An assassination attempt is contrary to everything we stand for as a nation. Everything. It’s not who we are as a nation. It’s not American. And we cannot allow this to happen,” Mr Biden said.

“Unity is the most elusive goal of all, but nothing is more important than that right now.”

Mr Biden plans to address the nation on Sunday in a prime-time speech from the Oval Office.

President Joe Biden salutes as he boards Air Force One
President Joe Biden salutes as he boards Air Force One on Saturday (Manuel Balce Ceneta/AP)

The president also said he’d had a “short but good conversation” with Mr Trump on Saturday and that he was sincerely grateful that Mr Trump is “doing well and recovering”.

Mr Trump, who called for national resilience in the hours after the shooting, posted on his social media account after Mr Biden’s remarks, “UNITE AMERICA!”

Mr Biden’s campaign team, meanwhile, is grappling with how to manage the political implications of an attack on the man he hopes to defeat in the November election.

He postponed a planned trip to Texas on Monday, where he was to speak on the 60th anniversary of the Civil Rights Act at the Lyndon B. Johnson presidential library, the White House said.

Vice President Kamala Harris had joined Mr Biden for an update from top investigators, including attorney general Merrick Garland, department of homeland security secretary Alejandro Mayorkas, and FBI director Christopher Wray.

Also taking part were White House national security adviser Jake Sullivan and Kim Cheatle, director of the US Secret Service.

Mr Trump himself called for unity and national resilience, and aides said he was in “great spirts and doing well” after being injured during the rally in Pennsylvania.

He is pushing ahead with plans to attend this week’s Republican National Convention in Milwaukee, where criticism of Mr Biden and the Democrats is sure to be searing.

In the moments after the shooting, Mr Biden’s re-election campaign said it was putting a hold on “all outbound communications and working to pull down our television ads as quickly as possible”.

It was not clear how long the suspension would last.

Investigators are still in the early stages of determining what happened and why.

But some are criticising Mr Biden for telling donors in a private call on July 8 that “it’s time to put Trump in the bullseye”.

In the donor call, Mr Biden said: “I have one job and that’s to beat Donald Trump. I’m absolutely certain I’m the best person to be able to do that.”

He continued: “So, we’re done talking about the debate. It’s time to put Trump in the bullseye. He’s gotten away with doing nothing for the last 10 days except ride around in his golf cart, bragging about scores he didn’t score. Anyway I won’t get into his golf game.”

In his initial response to the shooting on Saturday night, Mr Biden condemned the attempt on Mr Trump’s life. The White House also said then that the two men spoke, but did not release details.

“Look, there’s no place in America for this kind of violence,” Mr Biden said on Saturday, returning the White House from Delaware, where he had been spending the weekend.

“It’s sick. It’s sick. It’s one of the reasons why we have to unite this country. We cannot allow for this to be happening. We cannot be like this. We cannot condone this.”