Hillary Clinton’s unsecured home server contained more than a dozen emails deemed “top secret”- one of the highest levels of classification in the US government, the White House has said.
State Department spokesman John Kirby said the documents were not marked classified at the time they were sent.
Mrs Clinton’s use of a personal email as secretary of state has become an issue in her presidential campaign.
This is the first time her emails have been labelled classified at any level.
Messages were marked “top secret” because they would cause “exceptionally grave” damage to national security if disclosed, the State Department said.
Intelligence officials told the Associated Press that the 37 pages being withheld concerned so-called “special access programmes” – clandestine projects like drone strikes or government eavesdropping.
Hillary Clinton’s campaign said the emails were not marked classified when they were sent and should not be withheld.
“This appears to be over-classification run amok,” the campaign said in statement.
It was unclear whether Mrs Clinton sent “top secret” message or only received the information.
Previously, sensitive information had been redacted from Mrs Clinton’s messages, but Mr Kirby said the “top secret” emails would not be released, even in part.
Mrs Clinton’s opponents have accused her of putting US security at risk by using an unsecured computer system.
The presidential hopeful has admitted that her decision to use a private email server at her New York home was a mistake.
The State Department will release another batch of Mrs Clinton’s emails on Friday evening.
The department has yet to release about 7,000 pages of emails from Mrs Clinton’s private server.
Officials in the State Department have asked for additional time to vet the messages because of the recent snowstorm that hit Washington.
They have asked to release the final batch messages on 29 February, which is after the Iowa caucuses and the New Hampshire primary.