WASHINGTON: Hillary Clinton told the FBI she relied on her staff not to send emails containing classified information to the private email server she used as secretary of state.

The revelation came Friday as the FBI, in a rare step, published scores of pages summarising interviews with Clinton and her top aides from the recently closed criminal investigation into her use of a private email server in the basement of her Chappaqua, New York, home.

The Democratic presidential nominee told the FBI she never sought or asked permission to use a private server or email address during her tenure as the nation's top diplomat from 2009 to 2013.

A prior review by the State Department's internal watchdog concluded the practice violated several polices for the safekeeping and preservation of federal records.

The latest developments highlight competing liabilities for Clinton.

Either she made a conscious effort to prevent a full public accounting of her tenure at State or she was nonchalant about decisions with national security consequences and risks.

The first scenario plays into Republican arguments and voter concerns about her trustworthiness and transparency, while the second casts doubt on her pitch as a hyper-competent, detail-driven executive.

Clinton campaign spokesman Brian Fallon said Friday the campaign was pleased the FBI had released the documents.

"While her use of a single email account was clearly a mistake and she has taken responsibility for it, these materials make clear why the Justice Department believed there was no basis to move forward with this case," Fallon said.

GOP presidential nominee Donald Trump countered that Clinton's "answers to the FBI about her private email server defy belief".

"After reading these documents, I really don't understand how she was able to get away from prosecution," Trump said in a statement.

Clinton has repeatedly said her use of private email was allowed. But over a 3½-hour interview in July, she told investigators she "did not explicitly request permission to use a private server or email address", the FBI wrote.

Clinton said no one at the State Department raised concerns during her tenure, and she said everyone with whom she exchanged emails knew she was using a private email address.

The documents also include technical details about how the private server was set up.

It is the first disclosure of details provided by Bryan Pagliano, the technology staffer who set up and maintained Clinton's IT infrastructure.

Pagliano secured an immunity agreement from the Justice Department after previously refusing to testify before Congress, invoking his constitutional right against self-incrimination.

Large portions of the FBI documents were censored.

The FBI cited exemptions protecting national security and investigative techniques. Previous government reviews of the 55,000 pages of emails Clinton returned to the State Department found that about 110 contained classified information.

Clinton and her legal team deleted thousands more emails she claimed were personal and private.

The FBI report details steps taken by Clinton's staff that appear intended to hamper the recovery of deleted data, including smashing her old Blackberry smartphones with a hammer and using special software to wipe the hard drive of a server she had used.

Friday's release of internal investigative documents by the FBI was a highly unusual step, but one that reflects extraordinary public interest in the investigation into Clinton's server.

The FBI focused on whether Clinton sent or received classified information using the private server, which was not authorised for such messages. Clinton told the FBI she relied on others with knowledge about handling classified files not to send her emails inappropriately.

Clinton said she was unfamiliar with the meaning of the letter "c'' next to a paragraph and speculated that it might be "referencing paragraphs marked in alphabetical order".

That particular email had been marked as classified at the confidential level, the lowest level of classification. Clinton said she did not pay attention to the level of classification "and took all classified information seriously", according to the FBI.

After a yearlong investigation, the FBI recommended against prosecution in July, and the Justice Department then closed the case.

FBI Director James Comey said that while Clinton and her aides had been "extremely careless" in dealing with sensitive materials, there was no evidence they intentionally mishandled classified information.

The FBI's review also found no direct evidence that Clinton's server was hacked but said her system would be a high-value target for foreign intelligence agencies and a sophisticated attacker would have been unlikely to leave behind evidence of a breach.

Clinton told the FBI she was unaware of specific details about the security, software or hardware used on her server.

Clinton also told the FBI she never deleted emails, nor instructed anyone else to do so, to avoid their potential release under the Freedom of Information Act.

However, the FBI report says Clinton contacted her predecessor, former Secretary Colin Powell, in January 2009 to inquire about his use of a BlackBerry.

Powell, who also used a private email account, warned Clinton that if it became "public" that she used a smartphone to "do business", her emails could become official government records subject to disclosure.

"Be very careful," Powell cautioned Clinton in an email. "I got around it all by not saying much and not using systems that captured the data."

Clinton said she later directed her aides to create a private email account and said it was "a matter of convenience" to use the home server shared with her husband, former President Bill Clinton.

She added that "everyone at State knew she had a private email address", though in separate interviews several on her team told agents they had no idea she was using a private account.