WASHINGTON (AFP) - Democratic presidential candidate Hillary Clinton apologised on Tuesday (Sept 8) for using a private e-mail server while she was secretary of state, calling the decision a "mistake".

The Democratic frontrunner, who is facing eroding support, has long dismissed the e-mail uproar as a manufactured imbroglio.

But in an interview on Tuesday evening, she told ABC's "World News Tonight" that she took full responsibility for the episode, which she regretted.

"That was a mistake. I'm sorry about that. I take responsibility," she told the news network.

She used her private e-mail account and home server in lieu of the official government e-mail system while she served as the top US diplomat from 2009 to 2013.

Until Tuesday, Mrs Clinton had demurred from issuing a direct apology.

"I certainly wish that I had made a different choice," she said last week, adding that she was "sorry" that the controversy was "confusing" for the public.

"As I look back at it now, even though it was allowed, I should have used two accounts, one for personal e-mails, one for work-related e-mails," she said.

The lingering e-mail saga has weighed on her popularity, with a Gallup poll showing her favorability rating at 41 per cent, compared to 51 per cent who view her unfavourably - the lowest level since 1992.

'NOT CLASSIFIED AT TIME'

And the issue is unlikely to go away anytime soon, since the Department of State, to which Mrs Clinton turned over 30,000 official e-mails in late 2014, is publicly releasing them to the public in batches.

Many contain information that has been retroactively classified, raising questions about whether Mrs Clinton was inappropriately sending and receiving highly sensitive material, and whether sufficient security measures were in place to protect her server from hackers.

She maintained that during her tenure at the State Department, she "did not send or receive any information that was marked classified at the time". She also added that "everyone" she had e-mailed with, including "people in the White House, of course across the State Department", knew that she was using a personal e-mail account.

"It appeared, you know, as my address," she said.

The Federal Bureau of Investigation is examining the server to determine whether the arrangement has compromised government secrets.

Amid rising discomfort over handling of the e-mail fallout, Vice-President Joe Biden, a fellow Democrat, has begun publicly discussing the prospect of his own White House bid.

Asked whether Mr Biden would make a good leader, Mrs Clinton gave her reluctant support for the man with whom she worked closely during the first term of the Obama administration.

"I think he could be a good president, there's no doubt about that," she answered.

But she took a harsh tone when it came to Republican frontrunner Donald Trump, whose ever-expanding lead atop his party's presidential field has made him a formidable force.

"He won't tell you how he would do anything," Mrs Clinton said.

Americans, she maintained, "need a leader who cares about them again. So - that's what I'm going to try to do".