US national security officials say Iran was responsible for sending threatening emails to Democratic voters.

The emails appeared to come from a far-right pro-Trump group and were meant to "incite unrest", National Intelligence Director John Ratcliffe said.

Mr Ratcliffe also said US officials found Iran and Russia had obtained "some voter registration information".

The announcement comes 13 days before the presidential election.

The unusual intelligence briefing this close to the vote is seen as a testament to the government's concerns over voting interference and disinformation campaigns from foreign actors.

Mr Ratcliffe said Iran's "spoof emails" claimed to be sent by the far-right Proud Boys group in order to "intimidate voters, incite unrest and damage" President Donald Trump.

He added that the voter data could be used in attempts to "communicate false information to registered voters that they hope will sow confusion chaos and undermine your confidence in American democracy".

Mr Ratcliffe said officials "have not seen the same actions from Russia", but are aware they have some voter information.

In many states, voter data is available upon request, though each state has different requirements on who can request voter information, what data is available and how this data might be used, according to the National Conference of State Legislatures.

"If you receive an intimidating or manipulative email in your inbox, don't be alarmed and do not spread it," Mr Ratcliffe said, calling the actions to influence US voters "desperate attempts by desperate adversaries".