Former US President Donald Trump was rushed off stage after gunshots erupted at a rally in Pennsylvania in an apparent assassination attempt.

Footage showed him grimace and raise a hand to his right ear, before ducking as several sharp cracks - a series of shots - broke out, the BBC reports.

He was quickly swarmed by US Secret Service agents and dragged off stage to a waiting vehicle. He raised a fist as he was bundled into the car.

The FBI have identified the dead gunman as Thomas Matthew Crooks, aged 20, from Bethel Park, Pennsylvania.

Blood was clearly visible on Trump's ear and face as protection officers rushed him away.

The suspect was shot dead at the scene by US Secret Service officers, the agency's spokesperson Anthony Guglielmi said. He added that one bystander was killed in the shooting and two others were critically injured.

Officials later revealed that all three victims were male.

Law enforcement sources told the BBC's US partner CBS News that Crooks had been armed with a rifle and had fired from an elevated structure a few hundred metres away outside the venue.

Earlier, agents told reporters in Butler they had yet to establish a motive for the assassination attempt.

Special Agent Kevin Rojek confirmed the agency was treating the shooting as an assassination attempt.