Republican presidential candidate Donald Trump has hammered his Democratic rival Hillary Clinton as a threat to the country, saying that electing her while the FBI is investigating material possibly related to her email set-up could throw the country into crisis.

Key points: Mr Trump says "nothing will get done" if Mrs Clinton is elected during FBI probe

Mr Trump says "nothing will get done" if Mrs Clinton is elected during FBI probe White House says it does not believe Mr Comey is trying to influence the election

White House says it does not believe Mr Comey is trying to influence the election Mrs Clinton tells supporters "there is no case here"

"The investigation will last for years. The trial will probably start," Mr Trump told a rally in Grand Rapids, Michigan.

"Nothing will get done. I can tell you, your jobs will continue to leave Michigan. Nothing's going to get done."

Mr Trump said electing Mrs Clinton on November 8 would leave the country "in a constitutional crisis" the country could not afford.

"Folks, we've got to get back to work. You can't take it, I can't take it, nobody can take it," he said.

Little is publicly known yet about the emails being investigated, other than that they were found as part of a separate probe into former Democratic US representative Anthony Weiner, the estranged husband of Huma Abedin, who is Mrs Clinton's closest aide.

FBI director James Comey told members of Congress on Friday the agency was probing more emails that might relate to Mrs Clinton's use of a private email server, but added "we don't know the significance of this newly discovered collection of emails".

On Monday (local time) the White House steered clear of direct criticism of Mr Comey, who was appointed by Democratic President Barack Obama in 2013.

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Mr Obama views the FBI head as a man of integrity and does not believe he is secretly trying to influence the outcome of the election, White House spokesman Josh Earnest told reporters.

However, he added that Mr Obama believed that regarding the FBI it was important for norms and traditions about making information public to be followed.

"He's in a tough spot, and he's the one who will be in a position to defend his actions in the face of significant criticism," he said.

US Attorney-General Loretta Lynch did not order Mr Comey to withhold from Congress the discovery of emails potentially related to the investigation.

She did, however, make it known that she thought his decision violated department policy, a source familiar with the matter told the Reuters news agency on Monday.

Will the email controversy hurt Clinton's support?

Sorry, this video has expired Trump thanks Anthony Weiner as attack over Clinton emails continues

Mr Trump has seized on the news to press his long-standing charge that Mrs Clinton lacks integrity, hoping he can make a late comeback and win the White House.

Opinion polls have shown Mrs Clinton's lead over Mr Trump was narrowing slightly since early last week.

It is not yet known if the email controversy will hurt her support. Millions of Americans have already cast their ballot in early voting.

While campaigning in Ohio, Mrs Clinton tried to move beyond the controversy, telling supporters to keep focused on winning the election.

"There is no case here," she said at a rally at Kent State University.

Earlier, she told a restaurant patron at a cafe in Cleveland that the email affair was "lots of noise".

"Lots of distractions. Throwing stuff at me. And we've just got to keep people up, moving and voting," she said.

Sorry, this video has expired 'There is no case': Clinton slams FBI email inquiry

At the rally, she cast Mr Trump as a threat to American national security because of his lack of experience, bad temper and thin skin, his praise for Russian President Vladimir Putin, and the fights he has picked with US allies in Europe and Asia.

Democrats have demanded that Mr Comey and the FBI rapidly work to make public what they know about the new email trove.

A source familiar with the matter said on Sunday that the FBI had secured a warrant to examine the emails.

Democrats say FBI concealing damaging information about Mr Trump

The FBI spent a year investigating Mrs Clinton's use of a private email server, instead of government systems, while she was secretary of state from 2009 to 2013.

Mr Comey concluded in July that while she and her staff had been "extremely careless" in handling classified information, there were no grounds for any charges.

Democrats have urged James Comey to release more details about the emails. ( Reuters: Jonathan Ernst )

The Clinton campaign and its supporters have furiously attacked Mr Comey for releasing information that raised fresh questions but provided no details so close to election day, and before he even knew whether the material was significant or relevant.

Some party leaders said the agency was concealing damaging information about the Mr Trump campaign.

US Senator Harry Reid, the Democratic leader in the Senate, accused him of "a disturbing double standard for the treatment of sensitive information, with what appears to be a clear intent to aid one political party over another".

Reuters