U.S. authorities claim Iran and Russia have registration information for some voters in the country.
According to the agency, Iran reportedly sent threatening emails to Democratic party voters.
National Intelligence Director John Ratcliffe said all the emails were 'sent through' a pro group to US President Donald Trump and right-wing Proud Boys.
Iran is said to have hacked the Proud Boys email and subsequently used the group's email address to intimidate voters.
According to Ratcliffe, the email was sent to provoke 'unrest', intimidate voters and tarnish Trump's image.
He added that voter data was likely to be used to spread false information about the US democratic system.
Ratcliffe also informed Russia that it was "still silent" even though it was aware that the country also had US voter data.
"If you receive a scary email in your inbox, do not be afraid and do not spread it," said Ratcliffe.
Meanwhile, the Director of the Federal Bureau of Investigation (FBI), Christopher Wray, stressed that the US electoral system is still safe and sound.
