FBI Director James Comey says it would have been "catastrophic" to conceal his decision to reopen an investigation into Democratic candidate Hillary Clinton's emails just 11 days before the 2016 presidential election.

In an appearance before the Senate Judiciary Committee on Wednesday, Comey said it made him "nauseous" to think that his announcement on October 28 may have affected the election's outcome.

It was Comey's most impassioned defence yet of his decision to tell Congress that the FBI had uncovered a new trove of Clinton-related emails.

Clinton said on Tuesday her election bid was derailed by Comey's letter to Congress about the probe of her use of a private email server while she was secretary of state, and by the WikiLeaks release of her campaign chairman John Podesta's emails, allegedly stolen by Russian hackers.

Comey told the panel he felt he had to speak out last year about the email probe because he had repeatedly told Congress the investigation was over.

"To not speak about it would require an act of concealment in my view," Comey said. "Concealing, in my view, would be catastrophic."

The FBI, which is supposed to remain politically neutral, said a few days later the new emails did not change its July decision not to recommend criminal charges against Clinton, but many Democrats believe the political damage was done.

Senators on Wednesday asked Comey why he decided to go public with the investigation of Clinton but not the investigation of whether Republican Donald Trump or his associates had inappropriate contact with Russian agents.

Comey tried to convince senators he made the right choice.

"It makes me mildly nauseous to think that we might have had some impact on the election, but honestly, it wouldn't change the decision," he said.

US intelligence agencies determined in December that Russia was behind the hacking of Democratic emails in an effort to sway the election toward Trump.