FBI Director James Comey abruptly announced on Sunday that Hillary Clinton should not face criminal charges related to newly discovered emails from her tenure at the State Department, lifting a cloud of uncertainty that has shadowed the final days of her presidential campaign.

In a letter to congressional lawmakers two days before Election Day, Comey said the FBI has worked "around the clock to process and review a large number of emails" obtained from a device belonging to Anthony Weiner, the disgraced former congressman and estranged husband of top Clinton aide Huma Abedin.

Comey said the review has not changed the bureau's assessment from earlier this year that Clinton should not be prosecuted for her handling of classified information at the State Department.

Clinton's campaign welcomed the FBI announcement.

"We're glad this matter is resolved," Jennifer Palmieri, Clinton's communications director, told reporters traveling with the campaign to Ohio.

Clinton was infuriated by Comey's decision to alert Congress late last month that the FBI was reviewing new materials, calling it "unprecedented" and "deeply troubling". The decision shattered what had appeared to be Clinton's solid grip on the race and emboldened Republican Donald Trump.

Trump landed in Minnesota for a rally moments after Comey's announcement. He made no direct mention of the FBI decision and continued to insist - without evidence - that Clinton would be under investigation during her potential presidency.

"She's protected by a rigged system," he said. "She shouldn't even be allowed to run for president."

The FBI began investigating the handling of classified material on Clinton's private server in New York shortly after she announced her bid in April 2015. Last July, in an extraordinary public statement on an ongoing case, Comey announced that he was not recommending criminal charges against Clinton and called the decision "not even a close call".

But he also delivered blistering criticism of Clinton, calling her and her team "extremely careless" with her handling of national secrets.

Clinton had appeared to be heading for victory before the FBI review, but Comey's announcement blunted her momentum. Since then, national polls and battleground states have tightened, though Clinton still appears to hold an edge over Trump in the campaign's last moments.

During remarks at a black church on Sunday morning, Clinton urged voters to choose "unity over division". She warned that President Barack Obama's legacy is on the line, part of her strategy to shore up black voters who may be less enthusiastic about her than the president.

Trump opened a busy day of campaigning in Iowa, the battleground state where he appears in the strongest position. He also planned to make stops in Minnesota, Michigan and Pennsylvania, three states that have reliably voted for Democrats in presidential elections, as well as Virginia, a state Clinton's campaign believes it has a solid hold on.