FBI Director James Comey said on Wednesday it made him feel "mildly nauseous" to think he had an impact on the 2016 U.S. presidential election, but that he had no choice but to announce just days before the vote that the agency had reopened a probe into Democratic candidate Hillary Clinton's emails.

Comey told the Senate judiciary committee that one reason for the FBI's renewed interest in Clinton's emails ahead of the Nov. 8 election was that investigators had found emails, some of which were classified, forwarded by Clinton's assistant Huma Abedin to her husband who was not authorized to see such information.

It was Comey's most impassioned defence yet of his decision to tell Congress in a letter last October that the FBI had uncovered a new trove of Clinton-related emails, an announcement that angered Democrats.

Clinton said on Tuesday that her election bid was derailed in part by Comey's announcement about the renewed probe of her use of a private email server while she was secretary of state.

She also said her bid to defeat Republican Donald Trump was damaged by the WikiLeaks release of her campaign chairman John Podesta's emails, allegedly stolen by Russian hackers.

Comey told the Senate 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 that the new emails did not change an earlier decision not to recommend criminal charges against Clinton, but many Democrats believe the political damage was done.

"I'm not on anybody's side," he said at one point.

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

'Mildly nauseous' about possible impact on election

Comey tried to convince senators he made the right choice.

He said the probe into whether Russian hacking activities and American citizens was a classified investigation in its early stages, far different from the almost completed email probe.

"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.

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

The FBI is investigating possible collusion between Trump surrogates and the Russian government.

On Wednesday, Comey said the investigation was ongoing and declined to comment on whether Trump, or anyone else, had been ruled out as a possible subject.

Comey said Russia represents "the greatest threat of any nation on Earth given their intention and capability" and that its government is still involved in American politics.

Meanwhile, Comey concurred with Patrick Leahy, Democratic senator from Vermont, that citizenship alone is not a reliable indicator of the "terror threat" a person may pose to the United States. It was an apparent rebuke of Trump's executive order banning travel from several Muslim-majority countries.

"Most of the people that I talked about that we have open cases on are American citizens," Comey told the panel.

Trump's restrictions on visa issuance to citizens of Syria, Iran, Libya, Yemen, Sudan and Somalia is on hold by federal court order.