A top aide to President-elect Donald Trump on Thursday refused to rule out prosecuting Democrat Hillary Clinton or investigating the foundation run by former president Bill Clinton.

Ex-New York mayor Rudy Giuliani, a likely member of the incoming president's cabinet, told CNN that it was a "tough decision" on whether a special prosecutor should be appointed to look into possible illegal Clinton activities, as Trump threatened to do on the campaign trail.

"It's been a tradition in our politics to put things behind us. On the other hand you have to look at how bad was it?" he said.

"Equal administration of justice. It's one of our most important principles," he said.

Trump in August called for a special prosecutor -- an independent outsider -- to probe the financial dealings of the Clinton Foundation, claiming while on the campaign trail that the FBI and the Justice Department could not be trusted to impartially investigate the case.

Trump suggested that the foundation allowed Clinton to set up a "pay for play" scheme while she was the US secretary of state.

Speaking on Fox, the ex-mayor said "I don't think President Obama should pardon her" -- even though Clinton has not been convicted of any crime.

Giuliani was coy when asked if he'd accept if offered the office of the US attorney general.

"I certainly have the energy, and there's probably nobody that knows the Justice Department better than me," he told CNN.

In the early 1980s, under president Ronald Reagan, Giuliani had a senior position in the US Justice Department. He was later US attorney for the southern district of New York, where among other things he prosecuted cases against the mafia.

Giuliani was New York mayor from 1994-2001, leading the city in the aftermath of the September 11 terror attacks.