U.S. intelligence agencies on Monday pointed fingers at Iran over the alleged hacking of former President and Republican presidential nominee Donald Trump's election campaign.

"We have observed increasingly aggressive Iranian activity during this election cycle, specifically involving influence operations targeting the American public and cyber operations targeting presidential campaigns," said a joint statement from the Office of the Director of National Intelligence, the U.S. Cyber ​​and Infrastructure Security Agency and the FBI.

This included the recently reported activities to compromise Trump's campaign, the statement said.

It is the first time the U.S. government has attributed responsibility for the cyberattack that Trump had previously linked to Tehran.

"Iran seeks to stoke discord and undermine confidence in our democratic institutions," the statement said.

"Iran has furthermore demonstrated a longstanding interest in exploiting societal tensions through various means, including through the use of cyber operations to attempt to gain access to sensitive information related to U.S. elections."

It was "important to note that this approach is not new," the statement added.

"Iran and Russia have employed these tactics not only in the United States during this and prior federal election cycles but also in other countries around the world."

"We will not tolerate foreign efforts to influence or interfere with our elections, including the targeting of American political campaigns."

In a report earlier this month, Google attributed the attacks to a group of cybercriminals with close ties to Iran.

The tech giant said the APT42 hacker group, linked to Iran's elite Islamic Revolutionary Guard Corps, accessed email accounts belonging to around a dozen Democratic and Republican election campaign staff in May and June.