A US newspaper reports that the FBI is investigating allegations related to Donald Trump's presidential campaign that it came under an Iranian cyberattack.

The Washington Post carried an article on Monday that the FBI confirmed the probe. It said over the weekend that a 271-page document vetting Trump's running mate J.D. Vance had been leaked.

It quoted US intelligence officials as saying that Iran was working to stoke societal discord and undermine Trump's bid to regain the White House.

The paper also says the FBI is investigating suspected hacking attempts by Iran targeting the Democratic Party campaign.

Trump posted on social media over the weekend that his campaign was informed by Microsoft that one of its websites was hacked by the Iranian government. His campaign also claimed that Vance's vetting document had been compromised.

Hostilities between the United States and Iran were heightened after Trump decided to pull the US out of the Iranian nuclear deal when he was in office in 2018. He also reinstated sanctions against Tehran.