WASHINGTON (BLOOMBERG) - A top US intelligence official warned that Iran and Russia are attempting to interfere with the presidential election, with Iran already spreading false information to American voters.

"We have confirmed that some voter registration information has been obtained by Iran, and separately by Russia," director of National Intelligence John Ratcliffe said in a hastily called press conference on Wednesday (Oct 21) evening.

"This data can be used by foreign actors to attempt to convey misinformation."

"We ask every American to do their part to defend against those who wish us harm," Mr Ratcliffe said. "Do not allow these efforts to have their intended effect."

The voter information obtained was publicly available, and no hacking was involved, according to a US official who asked not to be identified discussing the sensitive matter.

Iran has been sending threatening e-mails to Democratic voters that purport to be from the right-wing Proud Boys group, according to the official.

Mr Ratcliffe, who was joined by FBI director Christopher Wray, said Iran aimed to damage President Donald Trump's re-election effort.

"We have already seen Iran sending spoofed e-mails designed to intimidate voters, incite social unrest and damage President Trump," he said.

Before becoming intelligence chief, Mr Ratcliffe was a Republican congressman from Texas who fervently supported Trump's claim that he was the victim of a "witch hunt" by "deep state" opponents who sought to link him to Russian interference in 2016.

Neither Mr Ratcliffe nor Mr Wray took questions from reporters.

Mr Judd Deere, a White House spokesman, said in a statement that Mr Trump "has directed the FBI, DOJ, and defence and intel agencies to proactively monitor and thwart any attempts to interfere in US elections, and because of the great work of our law enforcement agencies we have stopped an attempt by America's adversaries to undermine our elections".

The administration rushed out the announcement - while the President was speaking at a campaign rally in North Carolina - because the interference was about to be reported by at least one news organisation, according to a person familiar with the matter, adding that White House officials wanted to control the narrative and seem transparent.

Another person said that the White House and Mr Ratcliffe made the decision.

Iran views Trump as an arch-enemy since he quit the multinational nuclear accord with the Islamic Republic, expanded economic sanctions and ordered the killing of one of its top generals.

Mr John Hultquist, a senior director at the cyber-security firm FireEye Inc, said in a statement that the episode "marks a fundamental shift in our understanding of Iran's willingness to interfere in the democratic process".

US stock futures dropped on the news, with S&P 500 Index contracts down 0.7 per cent at 9.30am Hong Kong time, while the dollar edged higher.

Local media in Florida first reported on Tuesday that Democratic voters were receiving hate mail threatening violence in messages that claimed to be from the Proud Boys.

The e-mails read "Vote for Trump or else!" in the subject line.

Although voter registration material is available publicly, the e-mails falsely asserted that they had the voters' personal information because local election systems had been compromised by hackers.

"You will vote for Trump on Election Day or we will come after you. Change your party affiliation to Republican to let us know you received our message and will comply," read one of the e-mails reviewed by the cyber research firm Proofpoint. "I would take this seriously if I were you."

In a joint statement, Mr Marco Rubio, the Republican chairman of the Senate Intelligence Committee, and Mr Mark Warner, the panel's top Democrat, said: "Our adversaries abroad seek to sow chaos and undermine voters' belief in our democratic institutions, including the election systems and infrastructure that we rely on to record and properly report expressions of the voters' will."

Mr Alireza Miryousefi, a diplomat at the Iranian mission in New York, said in a statement that his country "has no interest in interfering in the US election and no preference for the outcome. The US must end its malign and dangerous accusations against Iran".

Just before Mr Ratcliffe and Mr Wray announced the threat to the Nov 3 election, the Washington Post reported that Mr Trump is considering firing Mr Wray after the election.

Mr Trump has faulted him for not providing fuel for the President's contention that anti-Trump bias drove the FBI's investigation into whether anyone involved in his 2016 campaign colluded in Russia's interference.

Although Mr Ratcliffe made only passing reference to Russia in Wednesday's announcement, the US intelligence community reported in August that "Russia is using a range of measures to primarily denigrate" former vice-president Joe Biden, Mr Trump's Democratic opponent, and "Kremlin-linked actors are also seeking to boost President Trump's candidacy on social media".

The intelligence assessment also said that "Iran seeks to undermine US democratic institutions, President Trump, and to divide the country in advance of the 2020 elections".

In September, Facebook Inc and Twitter Inc found social media accounts linked to the Internet Research Agency - the Kremlin-linked troll farm that used a coordinated operation on social media in an effort to help the Trump campaign in 2016 - that were attempting to build an audience on the American left ahead of the 2020 vote.

Last month, Microsoft Corp reported that Russia and Iran had attempted to hack into political targets ahead of the Nov 3 election.

Mr Ratcliffe's announcement came the same week that US charged six current and former members of Russia's military intelligence unity over a series of destructive cyber attacks in recent years.