Democratic presidential frontrunner Hillary Clinton is now facing a criminal inquiry by the FBI regarding her use of a personal email account, a private server and other devices during her four years as secretary of state, reports said.

"It's definitely a criminal probe," a federal source told The New York Post on Wednesday. "I'm not sure why they're not calling it a criminal probe."

The source made it clear that if the FBI finds that a security violation was committed, it "would lead to criminal charges."

If proven that Clinton knowingly sent, received or stored classified information in an unauthorised location, she may be prosecuted under the same misdemeanor federal security statute used to prosecute former CIA Director Gen. David Petraeus.

Former federal prosecutor Bradley Simon said the statute is rarely used and would be subject to the discretion of the attorney general.

"They didn't hesitate to charge General Petraeus with doing the same thing, downloading documents that are classified. The threshold under the statute is not high — they only need to prove there was an unauthorised removal and retention" of classified material," Simon said, according to New York Post.

Clinton's camp confirmed on Thursday that it has been notified of the FBI investigation.

Clinton's lawyer David Kendall assured that they will actively participate in the inquiry, which aims to discover the extent the former senator relied on her home service and other private devices to send and store classified documents.

"Quite predictably, after the IC (intelligence community) IG (inspector general) made a referral to ensure that materials remain properly stored, the government is seeking assurance about the storage of those materials," Kendall told CNN.

The New York Post earlier reported that two IGs alerted the Department of Justice about potentially classified information having likely ended up in the server in Clinton's New York home.

About four out of 40 sample emails of Clinton's correspondence during her four-year stint as US Secretary of State that were reviewed contained classified information, the publication revealed.

According to NBC News, the FBI has in fact contacted the IT company that helped maintain Clinton's private server, as well as her personal lawyer, who has a copy of Clinton's emails on a thumb drive.

Clinton's campaign team downplayed the FBI's investigation, denying that it is a "criminal probe" and asserting that it just a fact-finding inquiry.

Clinton's spokesperson Nick Merrill insisted that the former US First Lady did not send or receive marked classified information on her personal email server.

"She did not send nor receive any emails that were marked classified at the time," Merrill said.

"We want to ensure that appropriate procedures are followed as these emails are reviewed while not unduly delaying the release of her emails. We want that to happen as quickly and as transparently as possible," he added.