Hillary Clinton has responded to the FBI's scathing assessment that she was "extremely careless" with classified government secrets, by shifting the blame onto her former colleagues at the US State Department.

After maintaining for more than a year that she did not send or receive classified information through her unauthorised private email server, Mrs Clinton acknowledged she may have at least unwittingly done so, three days after the FBI concluded this happened at least 110 times.

The presumptive Democratic presidential nominee said she "certainly did not believe" that she was handling classified information on her server at the time, but emphasised that she followed the lead of her subordinates on whether information was classified.

"These are experienced diplomats, they have expertise in handling classified material," Mrs Clinton said.

"They were not careless and the material that they sent, they did not believe that was classified."

Mrs Clinton, who was the department's most senior classifying authority during her four-year tenure at its helm, did not address the FBI's conclusion that she herself sent information on topics classified as "top secret", the highest level, through the private server she kept in her basement.

James Comey called Mrs Clinton and her staff "extremely careless". ( AFP: Yuri Gripas )

It is a crime to mishandle classified information, and while FBI director James Comey said there was evidence Mrs Clinton or her aides may have broken these laws, there was not enough evidence of criminal intent for a prosecution.

In an unusual 15-minute announcement explaining the FBI's findings, Mr Comey ended up dismaying both Republicans and Democrats.

While Mrs Clinton's Republican opponents have fumed at the decision not to file criminal charges, Mrs Clinton and her staff have disputed some of Mr Comey's criticisms that undermine her argument she has better judgement than businessman Donald Trump, the presumptive Republican presidential candidate.

Mr Comey called her and her staff "extremely careless" and said that any "reasonable" government employee should have known such information should not be aired in emails.

Her server was so poorly secured the FBI could not eliminate the possibility it had not been hacked by the country's enemies, Mr Comey said.

Asked if she agreed that she was "extremely careless", Mrs Clinton said she was not, adding that Mr Comey had "clarified" his remarks. It was unclear what clarification Mrs Clinton was referring to.

In lengthier comments before Congress, Mr Comey again spoke of Mrs Clinton's carelessness and "real sloppiness", adding that it seemed she was not "particularly sophisticated with respect to classified information".

Reuters