Hillary Clinton
Democratic presidential nominee Hillary Clinton addresses a rally at the University of North Carolina in Charlotte, North Carolina Robyn Beck/ AFP

The FBI is reopening its investigation into Hillary Clinton's private emails after the discovery of new messages, its director James Comey said.

Comey had previously closed its probe into the Democratic presidential candidate's handling of sensitive government data and her use of a private email server to communicate with staff and officials as Secretary of State.

But in a letter to members of Congress on Friday (28 October), he wrote: "In connection with an unrelated case, the FBI has learned of the existence of emails that appear to be pertinent to the investigation.

"I am writing to inform you that the investigative team briefed me on this yesterday, and I agreed that the FBI should take appropriate investigative steps designed to allow investigators to review these emails to determine whether they contain classified information, as well as to assess their importance to our investigation."

He added: "Although the FBI cannot yet assess whether or not this material may be significant, and I cannot predict how long it will take us to complete this additional work, I believe it is important to update your Committees about our efforts in light of my previous testimony."

Comey had said in July it would not recommend criminal charges following the conclusion of the original investigation into Clinton's emails, but called her handling of sensitive information "extremely careless".

The decision to reopen the probe comes just 11 days before the presidential election. The Clinton emails controversy has loomed over her campaign throughout, with Republican rival Donald Trump repeatedly calling for a new investigation.

Trump took to the stage in New Hampshire shortly after the news broke.

He told his supporters: "They are reopening the case into her criminal and illegal conduct that threatens the security of the United States of America.

"Hillary Clinton's corruption is on a scale we have never seen before – we must not let her take her criminal scheme into the Oval Office."

He added: "I have great respect for the fact that the FBI and Department of Justice are now willing to have the courage to right the horrible mistake that they made."

The controversy surrounds Clinton's use of a personal server and email address – hdr22@clintonemail.com – during her four years as secretary of state, from 2009 to 2013. She shunned the use of an official email account, which would have held the emails on a government server.

Clinton claimed she made the decision so she would only have to carry one smartphone and deal with one email account.

But critics claimed using a private server presented a security risk to sensitive government data and meant her messages would be free from freedom of information requests.

The Clinton campaign has yet to respond to the latest developments.