Hillary Clinton's campaign team has accused the FBI of "double standards" after its decision to launch a new probe into her emails slashed the candidate's election lead over Republican rival Donald Trump almost in half.

The Democrat's lead slumped fron 4.6 percentage points on Friday to 2.5 points late on Monday, according to the Real Clear Politics tracker which averages most major polls.

The FBI said on Friday that it had found thousands of emails, during an unrelated investigation, that may be "pertinent" to its review of Ms Clinton's use of a private server from 2009 to 2013.

But Clinton campaign manager Robby Mook decried what he called a "blatant double standard" following a CNBC report that FBI Director James Comey opposed releasing details about possible Russian interference in the US election because it was too close to Election Day. The FBI declined to comment on that report on Monday.

Mr Trump has been making political hay out of the FBI's new investigation, despite the fact Mr Comey's letter contained very little detail about what the FBI thinks it might have found.

"That's the motherlode," Mr Trump said at a rally in Grand Rapids, Michigan on Monday night. "I think you're going to find the 33,000 (emails) that are missing."

But Ms Clinton, at her own rally in Kent, Ohio, said the FBI was welcome to check her aide's emails too.

"They should look at them and I'm sure they'll reach the same conclusion they did when they looked at my emails for the past year: there is no case here," she said.

A Reuters survey conducted in the midst of the FBI's announcement showed Ms Clinton on 44 per cent of the vote to Mr Trump's 39 per cent, already slipping slightly from a six-point lead.

But that poll may not yet represent the full impact of the damage from Mr Comey's letter to Congress, that his agency was looking into new emails uncovered during an unrelated investigation into Anthony Weiner, the estranged husband of top Clinton aide Huma Abedin.

Rep. Weiner, the disgraced former New York congressman, is being investigated in connection with online communications with a teenage girl. The new emails were found on a computer he shared with Ms Abudin, one of Ms Clinton's closest advisors.

But with one week to go to election day, there is some doubt as to whether the FBI will be able to release the findings of its new probe before the nation goes to vote.

And Ms Clinton said that timing raised "a few questions" for the FBI. "It is pretty strange to put something like that out with such little information right before an election," she said.

"In fact it is not just strange, it is unprecedented and deeply troubling. Voters deserve to get full and complete facts, so we call on director Comey to explain everything right away and put it all out on the table."

The Justice Department, moving to address concerns over the timing of the revelation of the emails and a potential post-election spillover, said it would "dedicate all necessary resources" to concluding the review promptly.

At the White House, spokesman Josh Earnest said he would neither defend nor criticise the timing of Comey's disclosure. But he also said President Barack Obama does not believe Comey was trying to influence the election, or attempting to benefit one candidate or party.