The FBI is reopening an investigation into e-mails sent by Hillary Clinton via a personal server, its director has announced.

ITV News Washington correspondent Robert Moore reports:

The FBI had announced in July that its investigation into the Democrat presidential candidate's private e-mails server had concluded.

Director James Comey criticised Mrs Clinton as "careless", but said no charges should be brought.

But in a letter sent to government officials, Mr Comey said new e-mails had been discovered in a different case - and while it does not necessarily mean a reopening of the criminal investigation, it will involve "additional work" by investigation.

In connection with an unrelated case, the FBI has learned of the existence of e-mails that appear to be pertinent to the investigation … James Comey, FBI director

He went on to say that there is no indication yet as to whether the material would prove "significant", and does not know how long the additional scrutiny will take.

And he specified the new material did not come from Clinton's private computer at her home, but found on a separate device during a sexting probe into former congressman Anthony Weiner.

Clinton avoided the subject herself during a rally in Iowa on Friday, instead using her speech to describe Donald Trump as the "poster boy of bad business".

Robert Moore: Could FBI announcement cost Clinton the election?

In response, Clinton's campaign chair John Podesta urged the FBI to "immediately provide the American public more information than is contained in the letter" already sent to Republican committee chairmen.

He added that it was "extraordinary" such a move would be made 11 days before the election.

"The Director owes it to the American people to immediately provide the full details of what he is now examining", Mr Podesta said.

"We are confident this will not produce any conclusions different from the one the FBI reached in July".

Meanwhile, the Senate judiciary committee said it had not been expecting the announcement, and it had not been in response to any question from Republican party members on the committee.

Judiciary committee Republican spokesman Taylor Foy said they have no additional information concerning to what e-mails Mr Comey's letter was referring.

Republican presidential candidate Donald Trump wasted no time in seizing the opportunity to attack his Democrat rival, claiming her behaviour had been "criminal" and calling it a scandal "bigger than Watergate".

""They are reopening the case into her criminal and illegal conduct that threatens the security of the United States of America," he said.