The U.S. State Department has so far identified 305 emails from Hillary Clinton's private server used while she was secretary of state to be reviewed for potentially classified information, the agency said in a court filing on Monday.

It said the emails referred for review came from a sample of about 20 percent of the Clinton emails screened to determine if they can be released publicly.

Last December, Clinton, the front-runner for the Democratic Party nomination in the 2016 presidential election, handed over about 30,000 emails she sent and received while America's top diplomat.

Clinton's use of her private email while secretary of state from 2009 to 2013 came to light in March and drew fire from political opponents who accused her of sidestepping transparency and record-keeping laws.

The FBI was looking into the security of the federal records and classified information contained among Clinton's emails. The U.S. government considers federal records to be government property.

The Justice Department has said the FBI investigation began after a government watchdog said at least four emails out of a sample of 40 he inspected contained classified information, including two that contained information deemed "top secret," the highest classification level.

Clinton has said she did not send or receive any secret information using that account.

The government forbids the sending of classified information outside unsecured networks because it could harm national security if intercepted.

After months of pressure, Clinton last week gave the FBI her private email server and a thumb drive of work-related emails from her tenure.

Asked about the additional emails being reviewed by intelligence agencies, State Department John Kirby told a media briefing on Monday: "It's a healthy thing."

"It doesn't mean that all 300 are going to end up at some level of (classification upgrade). I suspect some will and I suspect some won't," Kirby said.