Multiple US Secret Service agents have been placed on leave following the assassination attempt against Republican presidential candidate Donald Trump, US media reported Friday.

The agents put on leave include members of the Pittsburgh field office, which coordinated security for Trump's July 13 campaign event in Butler, Pennsylvania, RealClearPolitics and several US TV networks said.

The Secret Service declined to comment on what it described as a "personnel matter" but said it is "committed to investigating the decisions and actions of personnel related to the event in Butler."

"The US Secret Service's mission assurance review is progressing, and we are examining the processes, procedures, and factors that led to this operational failure," spokesman Anthony Guglielmi said in a statement.

"The US Secret Service holds our personnel to the highest professional standards, and any identified and substantiated violations of the policy will be investigated by the Office of Professional Responsibility for potential disciplinary action," he said.

Secret Service Director Kimberly Cheatle resigned last month after acknowledging that the agency had failed in its mission to prevent the assassination attempt by a 20-year-old gunman.

Trump was wounded in the ear, two rally attendees were seriously injured and a 50-year-old Pennsylvania firefighter was killed before a Secret Service sniper shot the gunman dead.

Acting Secret Service chief Ronald Rowe pledged during an appearance before a joint Senate committee last month that the agency will discipline any agents found to have committed policy violations.

Trump, 78, held his first outdoor rally since the shooting on Wednesday, addressing a campaign event in Asheboro, North Carolina, from behind a pane of bulletproof glass.