The White House declined to comment but said there were no indications that the suspected shooter in Saturday's attempted assassination of Trump had any foreign or domestic accomplices.

Iran said the accusations against it were "unsubstantiated and malicious".

US officials have for years worried that Iran would retaliate against Trump for his ordering of the January 2020 killing of Iranian military commander Qassem Soleimani.

"As we have said many times, we have been tracking Iranian threats against former Trump administration officials for years, dating back to the last administration," Adrienne Watson, spokeswoman for the White House National Security Council, said.

"These threats arise from Iran's desire to seek revenge for the killing of Qassem Soleimani. We consider this a national and homeland security matter of the highest priority," she said.

CNN reported that the intelligence about the Iranian plot was passed on by a human source.

A US official said that on learning of the increased threat, the National Security Council contacted the Secret Service, which added resources and assets for Trump's protection.

The Trump campaign was also made aware of an evolving threat, the official said.

In a statement to Reuters, Iran's mission to the United Nations in New York said, "These accusations are unsubstantiated and malicious".

"From the perspective of the Islamic Republic of Iran, Trump is a criminal who must be prosecuted and punished in a court of law for ordering the assassination of General Soleimani. Iran has chosen the legal path to bring him to justice," Iran's statement said.

The Secret Service has been sharply criticised since a 20-year-old gunman who shot at Trump on Saturday was able to access a roof overlooking the rally and open fire, grazing Trump's ear, killing a man in attendance and wounding two other attendees.

With AAP.