'Top Secret' emails on Clinton server

"Top secret" material was sent through Hillary Clinton's private email server during her tenure as secretary of state, it was revealed Friday, just days before voters cast their first ballots in the presidential campaign.

State Department spokesman John Kirby said the emails, which he described as "22 documents covering 37 pages" from seven email chains during Clinton's tenure as secretary of state, would therefore not be released publicly.

Another 18 emails, from eight email chains, sent between then secretary Clinton and President Barack Obama will also not be released. But Kirby said those exchanges did not contain classified information.

Although emails previously released by the State Department have been partially redacted due to the nature of the information they contained, this was the first time entire messages were withheld.

The revelation about the top secret emails comes three days before Clinton -- the frontrunner for the Democratic presidential nomination -- goes to battle in the Iowa caucus, the first time the public will cast ballots on the long road to Election Day in November.

"These documents were not marked classified at the time they were sent," Kirby told a news briefing, explaining that the emails had been reviewed prior to public release and found to contain top secret information.

"The documents are being upgraded at the request" of US intelligence agencies. 

Senator Dianne Feinstein, the vice chair of the Senate Intelligence Committee, said that "none" of the email chains originated with Clinton or contained the mandatory markings that are required to accompany classified information when shared.

She described the material as being contained in 22 separate emails.

"The only reason to hold secretary Clinton responsible for emails that didn't originate with her is for political points, and that's what we've seen over the past several months," Feinstein added.