WASHINGTON: The US State Department on Monday made public the final batch of emails taken from a private server which former secretary of state Hillary Clinton controversially used during her time in office.

Clinton will hope the release will quiet the furore over her decision to spurn a government email account, but federal investigators are still probing whether her home-brew set-up posed a risk to national security.

The email scandal has been seized upon by Clinton's Republican opponents and is one of the few major clouds still looming over her otherwise very promising campaign to become the 2016 Democratic presidential nominee.

State Department spokesman John Kirby said the final batch of roughly 3,800 pages of mails brought the sum total released to more than 52,000 vetted and in some cases partially redacted pages of official correspondence.

When, last year, it emerged that Clinton had used a private server and non-official address for all her email while in her former post, her rivals cried foul, suggesting she may have been illegally covering something up.

Clinton protested that none of the mails had been marked "classified" when she sent them and, after her own lawyers had removed mails they deemed purely personal, submitted a 52,000 page document dump to the State Department.

Top secret

Over recent months, government lawyers have combed through the stack, retroactively investigating whether any mail contained information that should have been classified ─ and they found several that raised questions.

Some 22 have since been deemed to contain "top secret" information and more than 2,000 confidential information of a lower level of classification, prompting protests from Clinton's camp about excessive government secrecy.

Kirby said the final batch did not contain any top secret files and, indeed, that one mail about North Korea's nuclear program that United States (US) intelligence had argued should be deemed as such had finally been downgraded.

"Based on subsequent review, the intelligence community revised its earlier assessment," Kirby said, implying a tactical victory in the turf war between federal agencies over the sensitivity of the email trove.

"As we've noted before, the information available to diplomats and the judgements they form do not necessarily need to be classified just because there are parallel intelligence sources," he argued.

One more mail included in an email chain between Clinton and President Barack Obama was also removed from the final batch before publication, not because it was deemed secret but because it now forms part of the White House record.

Beyond that, Kirby said, he could only confirm that "at the request of a law enforcement agency" one one-page document is being withheld for legal reasons he was not able to disclose.

The remaining emails were posted on the State Department website, some of them with redactions. Monday's conclusion of the State Department releases will not curtail the ongoing broader investigation into the server, nor will it silence Clinton's vocal election year foes.

Reince Priebus, chairman of the Republican National Committee, declared Clinton had "recklessly jeopardised our national security and sensitive diplomatic efforts" and the scandal ought to disqualify her from the presidency.