Hilary Clinton
Hilary Clinton's personal emails have been vetted by The State Department Steve Pope/Getty

The US State Department says that 22 emails on Presidential hopeful Hillary Clinton's unsecured private server, have been marked as 'top secret'. This is the first time that any of her emails have been labelled as such and perhaps unsurprisingly, the agency has refused to discuss the subject matter contained within her correspondence.

In total seven email chains, covering 37 pages, are being withheld from scrutiny for containing material that could do "exceptionally grave" damage to national security if disclosed. The 'top secret' classification is the highest level of secrecy and The State Department spokesman John Kirby said the messages were not classified when they were sent.

The Associated Press said that the emails may have concerned drone strikes and so-called "special access programmes" or clandestine projects like government eavesdropping. Clinton's opponents have said that her unsecured home server has put the country at risk and the Democratic politician, herself, has admitted that her decision to use a private email server at her New York home was a misjudgement.

In just a few days, Clinton begins her race to the White House by facing the Iowa caucuses with the first votes cast for both the Democratic and Republican presidential nominees. "This appears to be over-classification run amok," the campaign said in statement.

The State Department have been searching through 55,000 of her messages with more than 1,000 messages marked at lower classification levels for public release. It has not been revealed whether she sent the "top secret" messages or only received the information.

Earlier in January, emails on Clinton's private server were flagged by two government agencies as containing classified information, including some on "special access programmes" that are above the "top secret" classification level. Officials said the emails included relatively "innocuous" conversations by State Department officials about the CIA drone programme, NBC News reported.

The State Department has asked for additional time to vet Clinton's messages because of the impact of Storm Jonas on Washington DC on 23 and 24 January. They are intending to release the final batch messages on 29 February after the Iowa caucuses and the New Hampshire primary.