FBI Director James Comey told members of Congress on Friday that investigators had "learned of the existence of emails that appear to be pertinent to the investigation" into Hillary Clinton's use of a private email server while secretary of state.

"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 wrote in a letter.

Comey did not provide any details about how long the FBI investigation might last, and said the bureau "cannot yet assess whether or not this material may be significant."

US officials quoted by "The New York Times" said the emails did not come from Clinton's private server.

Instead, they were reportedly found on a separate device belonging to longtime Clinton aide Huma Abedin and her husband, Anthony Wiener.

Jason Chaffetz, the Republican chairman of the House Oversight Committee, said Comey was reopening the case against Clinton, although the letter specifically states the new information will be reviewed.

Clinton has remained resilient in the wake of Comey's remarks. Speaking at a campaign stop in Iowa, she called for the FBI director to release all new information related to the probe.

"The American people deserve to get the full and complete facts immediately. The director himself has said he doesn't know whether the emails referenced in his letter are significant or not," Clinton told reporters.

"I'm confident whatever they are will not change the conclusion reached in July," she added.

'Bigger than Watergate'

At a campaign rally in New Hampshire, Republican presidential candidate Donald Trump said Clinton's "conduct threatens the security of the United States."  

Trump said he had "great respect" for the decision by the FBI and the Department of Justice. They were now "willing to have the courage to right the horrible mistake that they made" in concluding the investigation earlier, he said.

"We must not let her take her criminal scheme into the Oval Office," Trump said of Clinton, adding: "This is bigger than Watergate."

His campaign manager, Kellyanne Conway, said on Twitter that "a great day in our campaign just got even better."

Election Day approaching

Friday's revelation comes less than two weeks before the US election. The email controversy has dogged Clinton for much of the campaign, with Trump seeking to hold the issue up as an example of his rival's untrustworthiness.

Republican House Speaker Paul Ryan said Clinton "was entrusted with some of our nation's most important secrets, and she betrayed that trust by carelessly mishandling highly classified information."

Clinton has previously apologized for using the private email server, saying the arrangement was a mistake. Still, recent polls show she has retained her lead over Trump nationally and has made crucial gains in some swing states.

The FBI's initial yearlong inquiry found that Clinton had mishandled emails, but did not call for criminal charges to be brought against the former secretary of state.

ls, sms, nm, jm / cmk (AP, dpa, AFP)