Top Senate official Harry Reid says Federal Bureau of Investigation (FBI) director James Comey may have broken the law with revelations about Hillary Clinton's emails.

Reid made his accusation in a letter that was sent to Comey on Sunday, accusing him of double standards in his treatment of information about the rival candidates.

The Senate minority leader said that, while Comey had "rushed" to make details of the investigation into Clinton public, he had not done the same with information about Donald Trump's links with Russia.

"Your actions in recent months have demonstrated 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," the letter read.

"I am writing to inform you that my office has determined that these actions may violate the Hatch Act, which bars FBI officials from using their official authority to influence an election. Through your partisan actions, you may have broken the law."

'No danger to American interests'

The Congress veteran complained that the information in the Clinton emails was no different from material that was previously combed for evidence of illegality. Meanwhile, he said, the information about Trump's dealings with "a foreign interest openly hostile to the United States" had been kept under wraps.

"There is no danger to American interests from releasing it. And yet, you continue to resist calls to inform the public of this critical information," he said.

In September, Democrats asked the FBI to investigate whether Russia's government was attempting to undermine the 2016 US presidential election.

Reid finished with a note about his personal misgivings over Comey's appointment. "Please keep in mind that I have been a supporter of yours in the past… I led the fight to get you confirmed because I believed you to be a principled public servant," he said. "With the deepest regret, I now see that I was wrong."

The Clinton email inquiry was closed in July with no charges being filed. However, Comey dropped a bombshell on Friday with the announcement that his officials were reviewing a newly discovered trove of emails. It is unclear if the online communications, seized on a device belonging to Clinton aide Huma Abedin, were duplicates of ones already examined by law enforcement officers.

FBI obtains warrant

Abedin's communications were already being looked at as part of separate, unrelated "sexting" allegations against former New York Congressman Anthony Weiner.

The FBI obtained a warrant to trawl through the emails for new information on Sunday, according to US news networks.   Clinton has called for the FBI to release full details of its investigation into her affairs.

The email "scandal" has overshadowed Democratic candidate Clinton's campaign from the outset, but few experts believe the FBI will make significant progress before Election Day.

Clinton had opened a recent lead over Republican rival Donald Trump in national polls, after a series of gaffes by the billionaire and the publication of a video in which he is heard making lewd comments about women. An ABC News/Washington Post poll released on Sunday showed a statistically insignificant 1-point national lead for Clinton.

rc/tj (AP, AFP)