Milwaukee DC - Security measures for the US Republican Party convention and for Donald Trump have been stepped up following the assassination attempt on the former president.

Planning for the security of the huge gathering in Milwaukee, Wisconsin has been "reviewed and strengthened" following the assassination attempt on Saturday, the director of the Secret Service, Kimberly Cheatle, said on Monday.

The agency is under huge pressure after a catalog of failures left a gunman able to take a shot at Trump from a nearby rooftop during a campaign rally in Butler, Pennsylvania.

The plan for the security measures for the four-day party convention is flexible and will be further adapted if necessary "to ensure the highest level of security for convention attendees, volunteers and the city of Milwaukee," Cheatle added in a statement.

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Trump is set to be officially declared the Republican candidate for the presidential election in November at the party convention.

In addition to the increased security measures already implemented for Trump in June, changes will also be made "to ensure his continued protection for the convention and the remainder of the campaign," the statement added.

No details of the increased security measures were given.

US Homeland Security Secretary Alejandro Mayorkas promised an independent and thorough investigation to clarify how the attack could have happened.

"An incident like this cannot happen," Mayorkas told CNN on Monday. "When I say that something like this cannot happen, we are speaking of a failure."

Cheatle said that the Secret Service would work with all investigative authorities "to understand what happened, how it happened, and how we can prevent an incident like this from ever taking place again."

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It will also co-operate "fully" with an investigation ordered by President Joe Biden and possible congressional investigations.

Trump had also called for Secret Service protection for Kennedy "in light of what is going on in the world today."

"Given the history of the Kennedy Family, this is the obvious right thing to do!" Trump wrote on his Truth Social platform.

Mayorkas confirmed he had been directed to provide protection to the candidate.

Kennedy's uncle, President John F. Kennedy, was assassinated in Dallas, Texas in 1963. Five years later, his father, Robert, was shot to death in Los Angeles while on the campaign trail.