Watch live: FBI Director James Comey questioned by Senate Judiciary Committee

FBI Director James Comey said Wednesday he felt "nauseous" at the thought he swayed last year's US election by announcing he was reopening a probe into Hillary Clinton's emails just days before the vote.

But the FBI chief told a hearing of the Senate Judiciary Committee it would have been far worse to conceal his decision — which the Democrat Clinton claims was a key factor in her defeat to Donald Trump.

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

Comey shocked the country when he informed Congress he was reopening the probe into Clinton's unauthorized use of a private email server as secretary of state, months after declaring the probe found no evidence of criminal wrongdoing.

The about-turn followed the discovery of missing Clinton emails with classified material on the laptop of a former congressman, on Oct. 27 last year — barely 10 days before the election.

Comey says he was faced with two options: either conceal the investigation until after the Nov. 8 vote, or inform Congress. 

"Speak would be really bad. There's an election in 11 days. Lordy, that would be really bad," Comey said.

"Concealing in my view would be catastrophic."

Congress was duly informed, and the news leaked out immediately on October 28, casting a cloud over Clinton. 

The FBI was sharply attacked as taking a political stance and Democrats continue to bristle over Comey's actions.

In an interview on Tuesday, Clinton claimed that had been a major factor in her loss, saying: "If the election had been on October 27, I'd be your president." 

But Comey said it was the right choice and he would do it again if he had to.

"I've lived my entire career by the tradition that if you can possibly avoid it, you avoid any action in the runup to an election that might have impact, whether it's a dogcatcher election or president of the United States."

"Even in hindsight — and this has been one of the world's most painful experiences — I would make the same decision."