Latest national polling shows US presidential candidates Hillary Clinton and Donald Trump are neck-and-neck, with Mr Trump leading by just one point — well within the margin of error.

It is Mr Trump's first lead in the American ABC News poll since May, though it is a poll that has been particularly volatile.

An average of national polls puts Mrs Clinton ahead by just on four points.

Mr Trump's most recent campaign stop was Pennsylvania, a critical state that he needs to win in order to secure a path to the White House.

Mrs Clinton is ahead in Pennsylvania and is preparing to make three campaign stops in Florida.

The latest poll comes as the Clinton campaign tries to contain the damage from FBI director James Comey's announcement of new emails that might pertain to Mrs Clinton's use of a private server while secretary of state.

A top aide to Mrs Clinton has urged the FBI to disclose what it knows about any ties between Mr Trump and Russia, accusing the law enforcement agency of unfairly publicising its inquiry into Mrs Clinton's email practices while staying quiet about Mr Trump.

Clinton campaign manager Robby Mook questioned why the FBI's director had not released any information about its investigation into Russia's role in the US election or ties to Mr Trump and his senior aides.

"If you're in the business of releasing information about investigations on presidential candidates, release everything you have on Donald Trump. Release the information on his connections to the Russians," Mr Mook said on CNN.

"They don't say a thing when it comes to Donald Trump and investigations against him, yet when it comes to Hillary Clinton, for some reason, they are more than happy to talk.

Analysis from Stephanie March in Florida: Donald Trump is ahead slightly here in Florida. It will be a key state in this election. He can't really win without it essentially, and if Hillary Clinton loses it, she's going to have to pick up a lot of other states that are currently in the swing and battleground category. I've been talking to supporters of Mrs Clinton here today who are absolute die-hards and this latest email scandal is not turning them away at all. A lot of these people are spending their days going door-to-door canvassing, trying to get people to come out and vote, and that's one of the interesting things here on the United States — you don't have to vote. It's about getting them from their houses to the polling booths, particularly in Florida where they have early voting. People have been knocking on doors, organising transportation, encouraging people to get out there, to vote, to vote early, and to vote for Mrs Clinton, who has many more volunteers on the ground than Mr Trump. But he's enjoying the power of free media that he's benefitted from so much in the past year-and-a-half. So, Florida is a key state and both of them are going at it different ways, and we'll have to keep watching. Stephanie March is one of the ABC's North America correspondents.

The FBI opened a preliminary inquiry into allegations that Mr Trump or his associates might have had questionable dealings with Russian individuals or businesses, but found no evidence to warrant opening a full investigation, according to sources familiar with the matter.

In an effort to shift the focus away from the latest trove of emails and back to Mr Trump's vulnerabilities, the Clinton campaign released a provocative new ad raising the specter of a possible nuclear war if Mr Trump is elected president.

Mrs Clinton emphasised the message that Mr Trump has a bad temper and an overly cavalier attitude toward nuclear weapons at rallies in the battleground state of Ohio yesterday.

Mr Trump, however, urged people who voted early for his Democratic rival to cancel their ballots and switch to him.

"This is a message for any Democratic voters who have already cast their ballots for Hillary Clinton and who are having a bad case of buyer's remorse, in other words you want to change your vote," Mr Trump told a Wisconsin rally.

"So if you live here or in Michigan or Pennsylvania or Minnesota, you can change your vote to Donald Trump," he said.

Several states, including those cited by Mr Trump, have a process to allow voters who cast early ballots to change their votes, either by submitting new ballots or showing up at their polling place on election day.

ABC/Reuters