Billionaire republican presidential hopeful Donald Trump took aim at Democratic front runner Hillary Clinton today, contending she could be "very damaged" by an email scandal swirling around the former secretary of state.

And amid reports that Vice President Joe Biden was actively exploring a run, Trump claimed that if Clinton could "get over" the controversy both would be equally tough challengers.

"I think they're the same. I think that Hillary may be very damaged, however ... because of the email thing," Trump told ABC's 'This Week' talk show.

"You look at General Petraeus, what's happened to him, how he's been destroyed. And what he did was a fraction of what she's done," he added.

"But I don't know, assuming she could get over that, which I just don't know how she possibly can, but if assuming she could, I would say it would be pretty equal."Clinton has been dogged on the campaign trail by her use of a private email account while serving as secretary of state.

She has agreed to turn over her private server to the Federal Bureau of Investigation, but has insisted that she "never sent or received any information that was classified at the time it was sent and received."

Ex-CIA chief David Petraeus -- credited by some with changing the course of the Iraq war -- was given two years' probation and fined USD 100,000 for providing classified secrets to his mistress. He resigned in November 2012.

A poll last week showed that Trump is growing increasingly competitive in a general election matchup against Clinton, trailing the Democratic frontrunner by just six points.

The CNN/ORC poll showed Clinton leading Trump by 51 per cent to 45 per cent, a dramatically more competitive race than July's 56-40 spread and June's 59-35.

The New York Times, meanwhile, reported that Biden met with Elizabeth Warren yesterday, further fuelling speculation that he was considering a run for the White House.

Warren, a Democratic senator from Massachusetts, has an enthusiastic, loyal following among liberals in her party.