Facing growing criticism over a massive security failure, the US Secret Service on Monday (Jul 15) vowed to cooperate with an independent review after a shooter was allowed to open fire on Donald Trump.

The 78-year-old former president was injured but survived an assassination attempt at a campaign rally in Pennsylvania on Saturday, a brazen attack that shocked a nation already deeply polarised ahead of the November election.

"The Secret Service is working with all involved Federal, state and local agencies to understand what happened, how it happened, and how we can prevent an incident like this from ever taking place again," the agency's director Kimberly Cheatle said in a statement.

"We understand the importance of the independent review announced by President (Joe) Biden yesterday and will participate fully," Cheatle added.

Trump was speaking at a rally in Butler, Pennsylvania when multiple bangs rang out.

He clutched his ear, with blood visible on his ear and cheek, then ducked to the floor as Secret Service agents swarmed onto the podium, surrounding him and rushing him to a nearby vehicle.

The shooter and a bystander were killed, and two spectators were injured.

Newly surfaced video backs up reports from witnesses that they had called out to police and physically pointed at the shooter as he lay on the roof preparing to open fire, the Washington Post reported on Monday.

The shots targeting Trump rang out 86 seconds after the first audible attempts to warn the police, the Post said, citing an analysis it did of video clips from the scene of the attack.

Biden ordered a full review of security at the rally, as well as at this week's Republican National Convention in Milwaukee, Wisconsin, where Trump became the party's presidential nominee.

He also ordered Secret Service protection for independent candidate Robert F Kennedy Jr, an environmental lawyer and long-time vaccine skeptic who has no chance of winning in November, but whose candidacy could potentially sway close contests in key swing states.