FBI says Russia and Iran are attempting to influence the US election by obtaining American voters' details
Iranian intelligence was behind recent emails threatening Democrats ahead of the American election and Russia has also obtained voters' information, the US government announced.
It came after registered Democrat voters in at least four battleground states, including Florida and Pennsylvania, received intimidating emails purportedly from the far-right militia group the Proud Boys.
The emails warned "You will vote for Trump on election day or we will come after you", and included the recipients' addresses.
John Ratcliffe, Donald Trump's Director of National Intelligence, said the emails had been "spoofed" and were actually from Iran.
They were intended to "incite social unrest and damage President Trump".
Experts suggested Iran was trying to make Mr Trump look bad by linking him to the Proud Boys.
Mr Trump was widely criticised for his response when asked to condemn the group in the first presidential debate.
Mr Ratcliffe said Iran had also distributed a video which suggested people could send in fraudulent ballots from abroad, a move designed to undermine confidence in the election.
He did not disclose how the Iranians had obtained the information of voters, or how the Russians might be using it.
It was not clear if any hacking of voter registration systems was involved.
The Iranian operation may simply have used email addresses obtained from state voter registration lists, which include party affiliation and home addresses, and can include email addresses and phone numbers.
The US would "impose costs and consequences" on foreign countries interfering with its election on November 3, Mr Ratlciffe said.
At a hastily announced press conference in Washington, he said: "We have confirmed that some voter registration information has been obtained by Iran, and separately by Russia.
"This data can be used by foreign actors to attempt to communicate false information to registered voters that they hope will cause confusion, sow chaos, and undermine your confidence in American democracy.
"We have already seen Iran sending spoofed emails designed to intimidate voters, incite social unrest and damage president Trump."
He added: "The intelligence community's role is to identify threats and allow the American people to understand information, or perhaps more accurately disinformation, they are seeing on the internet.
"We would like to alert the public that two foreign actors - Iran and Russia - have taken specific actions to influence public opinion relating to our elections."
He said the video Iran had distributed implied that individuals could cast fraudulent ballots even from overseas.
"This video, and any claims about such allegedly fraudulent ballots, are not true," he said.
"These actions are desperate attempts by desperate adversaries. Our election systems are resilient and you can be confident your votes are secure."
He added: "Although we have not seen the same actions from Russia we are aware they have obtained some voter information, just as they did in 2016.
"Rest assured we are prepared for the possibility of actions by those hostile to democracy.
"We are on top of this and will use the most powerful weapon we have to combat these efforts - the truth, information."
Mr Ratcliffe said he had spoken with Republicans and Democrats in Congress and there was "unanimity".
He added: "The president has instructed me to keep the public informed as appropriate. I will do that with transparency and candour."
Christopher Wray, director of the FBI, said: "We are not gong to tolerate foreign interference in our elections or any criminal activity that threatens the sanctity of your vote, or undermines public confidence in the election.
"Our election infrastructure remains resilient. You should be confident that your vote counts.
"Early unverified claims to the contrary should be treated with a healthy dose of scepticism."
He urged voters to be "thoughtful, careful and discerning consumers of information online".
Mr Wray added: "We are not going to let our guard down."
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