United States President Joe Biden on Sunday urged citizens to unite in the hour of crisis when a failed assassination bid was made on his predecessor Donald Trump and said it was time to "cool down" political rhetoric in the country.

In a prime-time national address from the Oval Office, "I want to speak to you tonight about the need for us to lower the temperature in our politics and to remember, while we may disagree, we are not enemies we are neighbours, friends, coworkers, citizens, and most importantly, we are fellow Americans."

Calling for national unity, he said, "We must stand together."

"Yesterday's shooting at Donald Trump's rally in Pennsylvania calls on all of us to take a step back, take stock of where we are and how we go forward from here," he said.

The President spoke for the third time in less than 24 hours after Trump was shot at during an election rally in Pennsylvania.

Biden said the motive of the shooter is not known yet. "We don't know his opinions or affiliations. We don't know whether he had help or support or if he communicated with anyone else. Law enforcement professionals, as I speak, are investigating those questions."

"A former president was shot and an American citizen killed for simply exercising his freedom to support the candidate of his choosing. We cannot we must not go down this road in America. We've travelled before throughout our history. Violence has never been the answer," he said.

"There is no place in America for this kind of violence, for any violence ever. Period. No exceptions. We can't allow this violence to be normalised," he said.

Disagreement is inevitable in American democracy, he observed.

"Political rhetoric in this country has gotten very heated. It's time to cool it down. We all have a responsibility to do that," the President said.

"It's part of human nature that politics must never be a little battlefield, or God forbid, a killing field. I believe politics ought to be an arena for peaceful debate, to pursue justice and to make decisions guided by the Declaration of Independence in our constitution," he said. -- PTI