The FBI has opened an investigation into allegations from the Trump campaign that it was targeted by hackers working for the Iranian government.

“We can confirm the FBI is investigating the matter,” the agency said in a short statement on Monday without specifically naming the former president or Iran.

A Trump campaign spokesman told the BBC the documents were illegally obtained by "foreign sources hostile to the United States".

Iranian officials have denied any connection to the hack and the US government has not formally accused Iran.

The FBI is also looking into whether Iranian hackers targeted the Joe Biden-Kamala Harris campaign, according to CBS News, the BBC's news partner, citing people familiar with the investigation.

The Trump campaign statement came one day after Microsoft released a report indicating that Iranian hackers targeted the campaign of an unnamed US presidential candidate in June.

Microsoft’s Threat Analysis Center (MTAC) said that the campaign was sent a spear- phishing email – a message designed to look trustworthy in order to get the target to click on a malicious link.

On Saturday, Trump said the hackers were "only able to get publicly available information".

The FBI began the investigations in early summer after both campaigns experienced attempted phishing schemes, CBS News reported.

According to the Washington Post, three staff members of the Biden-Harris campaign were also targeted by phishing emails in the days before President Joe Biden announced that he was quitting the race.

A Harris campaign spokesperson said in a statement to the media that the campaign "vigilantly monitors and protects against cyberthreats, and we are not aware of any security breaches of our systems".

The BBC has asked the Harris campaign for comment.

According to the Washington Post, the FBI probe into the hacking attempts was first opened in June.

Democratic Congressman Adam Schiff was among several lawmakers calling for the FBI to reveal what it knows about the hacking attempts.

He said that the US intelligence community “moved much too slow to properly identity the hacking and dumping scheme carried out by Russia” in 2016 and “should act quickly here”.

It comes after Democratic Congressman Eric Swalwell said he was requesting a briefing from the Department of Homeland Security on the "alleged hack of Trump's presidential campaign".

"Yes, Trump is the most despicable person ever to seek office. He also sought foreign hacking in a past election... But that doesn’t mean America ever tolerates foreign interference," he posted on X/Twitter on Saturday.

At campaign rallies in 2016, he asked Russia to hack his opponent in the race, former Secretary of State Hillary Clinton.

US security sources had previously warned of an Iranian plot to assassinate Trump, unconnected to last month’s attempted shooting in Pennsylvania.

And on Tuesday, the US justice department charged a Pakistani man alleged to have ties to Iran with plotting to assassinate US officials, potentially including the former president.