The United States on Monday accused Iran of launching cyber operations against the campaigns of both U.S. presidential candidates and targeting the American public with influence operations aimed at fanning political discord.

"We have observed increasingly aggressive Iranian activity during this election cycle," said a statement by the FBI, the Office of the Director of National Intelligence and the Cybersecurity and Infrastructure Security Agency, which oversees the defence of government computer systems.

The statement confirmed charges made earlier this month by the campaign of Republican presidential nominee Donald Trump that Iran hacked one of its websites, triggering an FBI investigation.

At that time, Trump said Iran was "only able to get publicly available information."

Iran, the U.S. statement said, also has targeted the campaign of Vice President Kamala Harris, who is to officially accept the Democratic Party's presidential nomination at this week's convention.

Iran's mission to the United Nations issued a statement calling the allegations "unsubstantiated and devoid of any standing.

"As we have previously announced, the Islamic Republic of Iran harbours neither the intention nor the motive to interfere with the U.S. presidential election."

The U.S. statement said Iran has conducted influence operations targeting the American public in a bid to fan political divisions and "cyber operations targeting presidential campaigns."

"This includes the recently reported activities to compromise former President Trump's campaign, which the IC (intelligence community) attributes to Iran," it continued.

It said the intelligence community is confident Iranian operatives using social engineering and other means "sought access to individuals with direct access to the presidential campaigns of both parties," the statement said.

Those activities included thefts and disclosures "intended to influence the U.S. election process," the statement added, without elaborating.