Washington DC - The Secret Service increased security for Donald Trump weeks ago after authorities learned of an Iranian plot to kill him, although it was not linked to the recent attempt on his life, US media reported Tuesday.

CNN reported that US authorities received intelligence from a "human source" on a plan by Tehran targeting the former president, causing protection to be boosted for Trump. Other US outlets also reported the plot.

But it was not connected to the shooting at a campaign rally in Butler, Pennsylvania, in which gunman Thomas Matthew Crooks opened fire at Trump, leaving the Republican wounded and killing a rally attendee, they said.

The US National Security Council said it had been "tracking Iranian threats against former Trump administration officials for years" as Tehran sought revenge for the 2020 killing of Revolutionary Guards commander Qasem Soleimani.

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"We consider this a national and homeland security matter of the highest priority," spokeswoman Adrienne Watson said in a statement.

The investigation into the Trump shooting on Saturday "has not identified ties between the shooter and any accomplice or co-conspirator, foreign or domestic," she added.

Watson referred questions on "additional measures that have been implemented in recent weeks" to the Secret Service and Department of Homeland Security.

Top Secret Service spokesman Anthony Guglielmi said they and other agencies were "constantly receiving new potential threat information and taking action to adjust resources as needed."

"We cannot comment on any specific threat stream other than to say that the Secret Service takes threats seriously and responds accordingly," Guglielmi added in a statement.

There was no immediate reaction from the Department of Homeland Security.

The report comes as the Secret Service faces intense scrutiny over the Butler shooting, with questions over how a gunman was able to open fire at Trump from an exposed rooftop some 500 feet away.

President Joe Biden has ordered an independent review of the agency's handling of the incident.