WASHINGTON (AFP) - The US State Department said on Thursday (July 7) that it will reopen its internal investigation into whether Democratic White House hopeful Hillary Clinton's use of a private e-mail account and server compromised her handling of classified material.

"We will aim to be as expeditious as possible, but we will not put artificial deadlines on the process," department spokesman John Kirby said, noting that the internal review could proceed now that the Justice Department investigation wrapped up with no charges filed against the former secretary of state.

The FBI advised earlier in the week that no charges be brought over Mrs Clinton's use of a private e-mail account and homebrew server during her time as secretary of state from 2009 to 2013, a recommendation that the Justice Department then followed.

Although the announcement took a huge weight off the presumptive Democratic nominee, FBI chief James Comey nonetheless said Mrs Clinton and her staff were "extremely careless in their handling of very sensitive, highly classified information".

Republicans, including Mrs Clinton's White House rival Donald Trump, have cried foul over the lack of charges, alleging that her influence as a prominent politician helped her escape criminal proceedings.

Mr Trump, who polls show faces an uphill battle against Mrs Clinton in November, has repeatedly accused her of masking her involvement in illegal activity by deleting more than 30,000 e-mail messages she claimed were personal and not related to her job as top US diplomat.

On Thursday, Mr Comey was forced to vigorously defend his recommendation before the House Committee on Oversight, which summoned him to testify.

Despite tense moments, Mr Comey, a Republican former prosecutor, maintained his cool throughout the marathon grilling that lasted 4 hours and 40 minutes.

Although Mr Kirby declined to provide specific information about the State Department's latest review, he said it would aim to be "as transparent as possible about our results, while complying with our various legal obligations".

Of some 30,000 e-mail messages Mrs Clinton turned over to the FBI, Mr Comey said 110 contained classified information - Mrs Clinton had said none was classified at the time they were sent.

Another 2,000 e-mail messages were later "up-classified" to confidential.