Hillary Clinton says she is "confident" a new FBI probe linked to her emails will not change its original finding that she should not be prosecuted.
The Democratic presidential candidate called on the FBI director to explain the new inquiry to the American people.
James Comey earlier said the FBI was looking into newly found messages
The latest emails came to light during a separate inquiry into top Clinton aide Huma Abedin's estranged husband, former congressman Anthony Weiner.
Devices belonging to Ms Abedin and Mr Weiner were seized in a separate FBI inquiry into whether he sent sexually explicit emails to a 15-year-old girl in North Carolina.
Mr Comey said investigators will determine whether the emails contain classified information.
At a news conference Mrs Clinton urged the FBI to release all new information from its investigation.
She said the American people deserve to get the full and complete facts from the fresh FBI investigation.
"The director himself has said he doesn't know whether the emails referenced in his letter are significant or not."
"I'm confident whatever they are will not change the conclusion reaching in July. Therefore it's imperative that the bureau explain this issue in question, whatever it is, without any delay."
The FBI spent about a year investigating the use of an unauthorised private email server between 2009 and 2013, when Mrs Clinton was Secretary of State, after it emerged that some of her emails contained classified government secrets.
Mr Comey has previously called Mrs Clinton's handling of classified information during that time as "extremely careless", but cleared her of any criminal wrongdoing.
The New York Times reported the new emails were discovered when the FBI seized electronic devices belonging to Mrs Clinton's aide Huma Abedin and her husband, the disgraced former congressman Anthony Weiner who resigned over a sexting scandal.
There was no immediate response from Mrs Clinton.
She has previously apologised for the use of her server, which was set up in her home in Chappaqua, New York.
She said the arrangement was a mistake, and that she did not knowingly send or receive classified information.
A US government source said the newly discovered emails were not found on that server but on a separate device during an unrelated investigation.
The chairman of the Clinton campaign, John Podesta, said the FBI should immediately provide more information about its discovery.
Trump jumps on new probe
News of the latest investigation was seized upon by Donald Trump, Mrs Clinton's rival for the presidency, who has repeatedly cited her email practices as disqualifying her for office.
"I need to open with a very critical breaking news announcement," Mr Trump said at the start of a rally in Manchester, New Hampshire, before describing Mr Comey's letter.
He was drowned out by a chant from the crowd: "Lock her up!"
"We must not let her take her criminal scheme into the Oval Office," Mr Trump continued. "I have great respect for the fact that the FBI and the Department of Justice are now willing to have the courage to right the horrible mistake that they made."
House of Representative Speaker Paul Ryan, who is the most senior elected Republican in Congress, said Mrs Clinton should not get classified briefings until the investigation was over.
"Yet again, Hillary Clinton has nobody but herself to blame," he said in a statement. "She was entrusted with some of our nation's most important secrets, and she betrayed that trust by carelessly mishandling highly classified information."
'Miss Finland' Trump grope accusation
Meanwhile a former Miss Finland has accused Mr Trump of groping her in 2006 when she was representing her country in the Miss Universe beauty contest.
Ninni Laaksonen has told a Finnish newspaper the Republican presidential nominee grabbed her behind before she appeared on a television show in New York.
The accusation is the latest in a stream of allegations of sexual assault against Mr Trump from other women in the United States.
Mr Trump has called his accusers liars.
- Reuters / BBC