US President Joe Biden has called for unity in a televised address one day after the attempted assassination of former President Donald Trump.

"I want to speak to you tonight about the need for us to lower the temperature in our politics," Biden said. "Do remember, though we may disagree, we are not enemies. We're neighbors. We're friends, co-workers, citizens. And most importantly, we are fellow Americans. We must stand together."

Biden also said he spoke with Trump and is grateful he was not seriously injured.

Gunfire erupted shortly after Trump took the stage at a campaign rally in the state of Pennsylvania on Saturday. One person in the crowd was killed, and two others seriously injured.

Secret Service agents killed the suspected gunman. The FBI has named him as Thomas Crooks, a 20-year-old Pennsylvania resident. They are investigating his motives.

US media say he fired from the rooftop of a factory building outside the venue, around 120 to 150 meters away from Trump.

Law enforcement officials are quoted as saying bomb-making materials were later found in the suspect's vehicle and home.

Crooks was apparently registered as a Republican voter and graduated from high school in Pennsylvania. ABC News says the rifle he used was legally purchased by his father.

Trump is scheduled to attend the Republican Party's convention, which kicks off in the state of Wisconsin on Monday.

He announced on social media that he would not delay his trip, saying he could not allow a "shooter or potential assassin" to change his scheduling.