WASHINGTON -- US House Oversight Committee Chairman Jason Chaffetz said on Friday the Federal Bureau of Investigation (FBI) has reopened Democratic presidential nominee Hillary Clinton's email investigation.

"The FBI has learned of the existence of emails that appear to be pertinent to the investigation. Case reopened," Chaffetz tweeted Friday.

According to a copy of an letter by FBI Director James Comey to Chaffetz cited by CNN, new emails had emerged recently that appeared to be linked to the FBI's Clinton email probe completed in July.

"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," said Comey in the letter.

However, Comey said the FBI "cannot yet assess whether or not this material may be significant", adding that he could not predict how long it would take investigators to complete the "additional work".

The Clinton campaign on Friday afternoon issued a statement urging the FBI to provide the public more information than is contained in the letter sent to the Congress.

"Already, we have seen characterizations that the FBI is 'reopening' an investigation but Comey's words do not match that characterization," said John Podesta, chairman of the Clinton campaign in a statement.

After a yearlong investigation, the FBI in July recommended no criminal charges against Clinton in its email probe, and the Justice Department then closed the investigation.

At a press conference in March 2015, Clinton acknowledged that she had exchanged about 60,000 emails from her private email account during her stint in the Obama administration, among which about half were personal and thus deleted.

All emails were sent and received via a private email server based at Clinton's home.

In response to requests from the State Department, the Clinton camp turned over the other half, roughly 30,000 emails in total, to the State Department in December 2014.

The controversy surrounding Clinton's email practices again burst into public view in August 2015 after the inspector general for the intelligence community revealed that two of the thousands of emails held by Clinton contained top-secret information. The revelation then trigged a federal investigation into whether Clinton had mishandled sensitive information.

Republican presidential candidate Donald Trump said Friday in a campaign rally that the FBI will "right the horrible mistake that they made".