US Democratic presidential nominee Hillary Clinton leaves the Cedar Park Cafe in Philadelphia, Pennsylvania, US Nov 6, 2016. [Photo/Agencies]

REPUBLICANS RENEW CRITICISM

The latest emails were discovered as part of a separate probe of former Democratic US Representative Anthony Weiner, the estranged husband of Clinton aide Huma Abedin. Weiner is the target of an FBI investigation into illicit text messages he allegedly sent to a 15-year-old girl in North Carolina.

Federal investigators got a warrant to examine the emails to see if they were related to the probe into Clinton's private server. Democrats reacted angrily to Comey's intrusion into the race and demanded quick action in examining the emails.

"I am very grateful to the professionals at the FBI for doing an extraordinary amount of high-quality work in a short period of time," Comey said on Sunday.

But Democrats did not let Comey and the FBI off the hook. US Senator Dianne Feinstein of California said Sunday's announcement made Comey's earlier letter "even more troubling" for creating a false impression about the inquiry.

"I believe the Justice Department needs to take a look at its procedures to prevent similar actions that could influence future elections," she said.

Trump, who has hammered Clinton over the emails issue, arguing it was proof she is corrupt and untrustworthy, did not mention the decision at a rally in Minneapolis right after it was announced.

Republican National Committee Chairman Reince Preibus said that while the probe had not led to criminal charges, it produced evidence that Clinton broke the law and "repeatedly lied to the American people about her reckless conduct."

Clinton also did not mention the FBI decision during a campaign appearance in Cleveland.

News of the renewed probe hurt Clinton's poll numbers, with Trump cutting into her once formidable lead.

The latest Reuters/Ipsos poll shows Clinton with a 5 percentage point lead over the New York businessman in the national survey - 44 percent to 39 percent support - while races in the swing states of Florida and North Carolina have shifted from favoring Clinton to being too close to call.

The Reuters/Ipsos States of the Nation project estimates that Clinton has a 90 percent chance of winning the election.