Washington: Material that should have been marked "top secret" has been found in seven email chains on former secretary of state Hillary Clinton's private server, according to the US State Department.

State Department spokesman John Kirby said the emails, which he described as "22 documents covering 37 pages," would therefore not be released publicly along with other emails from the Democratic White House candidate's controversial archive.

Kirby further added that the documents, totaling 37 pages, were not marked classified at the time they were sent, but are being upgraded at the request of the Intelligence Community because they contain sensitive information.

Eighteen emails, comprised of 8 email chains between Clinton and President Barack Obama, are being "withheld in full" to "protect the President's ability to receive unvarnished advice and counsel," Kirby said.

On Friday, Clinton did not mention the email controversy during campaign event. But in an interview with KCRG in Dubuque, Iowa, she maintained that she "never emailed anything that was considered to be classified" and asked for the emails to be released.

The State Department released over 900 Clinton's emails on Friday -- 242 of which received classification upgrades: 11 to "secret" and 209 more to "confidential," along with the 22 emails containing "top secret" information -- but the release fell behind the judge-imposed timetable for producing all of her emails.

Last Thursday, State Department filed a motion to extend the final productions until February 29 because the department had failed to send more than 7,000 pages of those emails to other government agencies for review, only recognizing the mistake earlier this month.

Florida Sen. Marco Rubio said Clinton's email use was a "disqualifier" for the White House. Moreover, Texas Sen. Ted Cruz told conservative radio host Hugh Hewitt that Clinton's email controversy would jeopardize her presidential aspirations.