Hillary Clinton's unsecured home internet server contained 22 emails with material that was top secret, the US government has confirmed.
The Democratic presidential contender has been the subject of a long-running controversy over her use of private email while she was Secretary of State.
She has insisted she never sent or received information on her personal email account that was classified at the time.
But the US State Department is withholding part of the latest batch of Mrs Clinton's emails, which were due to be released on Friday, as they are being "upgraded".
Seven email chains, constituting 37 pages of messages are being withheld.
John Kirby, a State Department spokesman, said of the emails "were not marked classified at the time they were sent", but had been withheld now at "at the request of the intelligence community".
The material in the emails was recently described by a key intelligence official as concerning so-called "special access programmes".
Those are a highly restricted sub-set of classified material that could point to confidential sources or clandestine programmes like drone strikes or government eavesdropping.
State Department officials would not describe the content of the emails or say if Mrs Clinton had sent any herself.
The email revelation comes just three days before the Iowa presidential nominating caucuses in which Mrs Clinton is a candidate.
None of Mrs Clinton's emails released so far have been marked "classified" or "top secret", but reviewers had previously designated more than 1,000 messages at lower classification levels for public release.