The US government has confirmed that former Secretary of State Hillary Clinton's home server contained closely-guarded government secrets, censoring 22 emails that contained material requiring one of the highest levels of classification.

The seven email chains from Clinton, who is also a Democratic nominee candidate in the 2016 presidential polls, will be withheld from the public as the information in them is classified at the highest level as "top secret", US Department of State Spokesman John Kirby said on Friday.

READ MORE: Clinton apologises for use of private email account

The announcement came three days before the Iowa caucuses, when the first votes are cast for the presidential nominations and where Clinton is locked in a tight race with Vermont Senator Bernie Sanders to become the Democratic nominee for the November election.

"These documents were not marked classified at the time that they were sent," Kirby said.

He added that the department was now investigating whether the information in them was classified at the time it passed through her private clintonemail.com email account run on a server in her New York home.

The department and intelligence officials have been arguing about the emails, which are being made public under a federal court order, for at least five months. The state department initially maintained that Clinton might have obtained the same information independently through nonclassified channels.

'Bureaucratic infighting'

The decision to no longer pursue this argument will add to the questions Clinton has faced for months over her handling of sensitive government information as she seeks to maintain her leading position in the Democratic race.

The Clinton campaign criticised the state department's decision as the result of "bureaucratic infighting" and "over-classification run amok", adding that the emails should be released.

Some information has been censored in more than 1,300 emails already made public because the state department says it is classified, including the privately shared thoughts of foreign leaders and government officials. But this is the first time that entire chains are being withheld.

Congressional Republicans have criticised and investigated Clinton for her use of a private email server for her work as a secretary of state between 2009 and 2013. Some of her Republican rivals for the presidency, including New Jersey Governor Chris Christie, have called for her prosecution.