Protection for Donald Trump was boosted several weeks ago after US authorities learned of an Iranian plot to kill him, according to national security officials.

Officials say there is no known connection between the alleged Iranian plot and the assassination attempt on the former president on Saturday in Pennsylvania.

However, the disclosure that security had been tightened raises further questions over how Thomas Matthew Crooks, 20, was able to climb a building and get close enough to fire at Trump.

The US Secret Service and the Trump campaign were notified of the Iranian threat, and security was increased as a result, according to a US national security official.

The Iranian mission at the United Nations called the report "unsubstantiated and malicious", adding that Trump was "a criminal who must be prosecuted and punished in a court of law".

Intelligence sources told CBS, the BBC's US news partner, that the Secret Service bolstered security in June in response to the Iranian threat. This included extra counter-assault and counter-sniper agents, drones and robotic dogs.

CBS reported that the details of a potential Iranian operation were obtained through "human source intelligence", and came amid a notable increase in Iranian chatter regarding attacks against Trump.

Trump and officials including his former secretary of state, Mike Pompeo, have faced threats from Tehran since ordering the drone strike assassination of Qassim Soleimani, the commander of Iran's Quds force, in Iraq in 2020.

Anthony Guglielmi, a spokesman for the US Secret Service, said it 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.”

The Trump campaign said it did not comment on security matters and referred BBC questions to the Secret Service.

Adrienne Watson, a spokeswoman for the White House National Security Council, said that US security officials had been "tracking Iranian threats against former Trump administration officials for years".

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

She however reiterated that the investigation "has not identified ties" between Crooks and "any accomplice or co-conspirator, foreign or domestic."

In 2022, the Department of Justice announced criminal charges against a member of Iran’s Islamic Revolutionary Guard Corps, alleging that he was orchestrating a plot to kill Mr Bolton.

Prosecutors said the plot was “likely in retaliation” for the killing of Soleimani.

Questions have swirled about how police officers and agents responsible for the rally at Butler County fairgrounds, Pennsylvania, allowed Crooks to get so close.

The director of the Secret Service admitted that local police were inside the building while Crooks was on its roof aiming at Trump 130m (430ft) away.

CBS News, the BBC's US partner, reported three local police snipers were inside the building and had seen Crooks getting on the roof.

The local sheriff's department referred BBC questions to the state police, which said it was not responsible for the area containing the building.

A state police spokesman told the BBC that it provided "all resources" requested by the Secret Service, including between 30 to 40 troopers inside the perimeter.

President Joe Biden has ordered an independent review of how the gunman could have come so close to killing Trump, and the Secret Service also faces probes from Congress.