The agency Clinton previously headed was set to release later some 7,000 additional pages of the mails she surrendered to US officials earlier this year after coming under fire for operating the server.
But, in a revelation that will embarrass Clinton as she campaigns for the Democratic presidential nomination, officials said many of these mails have now been ruled classified.
"I think it's somewhere around 150," State Department spokesman Mark Toner told reporters, adding that the process of reevaluating the remaining unreleased emails was continuing.
Last month, officials said that 63 other mails had also been "upgraded in some form."
Clinton has been criticized for using a private server rather than an official government domain for all her emails during her time at the State Department.
Critics allege that she used the so-called "homebrew server" to avoid political scrutiny of her time at State.
They also charge that she put national security at risk by taking classified information out of supposedly secure government systems and onto an unauthorized network that could be prey to hackers.
Clinton, for her part, insists none of the mails on the private server were formally marked "classified" or a higher designation such as "top secret."
Toner confirmed that the review, being overseen by the Intelligence Community Inspector General, has so far found this to be the case.
But, while the material reviewed so far was not marked "classified," the number of mails containing sensitive information that are now thought worthy of classification is increasing.
- 'Thorough scrub' -
Toner added that the latest declassification, added to previous such publications, brought the proportion of mails released or redacted as classified to more than 25 percent.
"The goal is we do a thorough scrub on whether these need to be redacted before they can be released publicly," he said.
Clinton sent or received a total of 62,320 emails during her term as secretary between 2009 and 2013.
She provided 30,490 official emails to the State Department for release to the public record, while declaring that her office has deemed the remaining 31,830 to be personal records.
Clinton's official correspondence, minus information deemed secret and then redacted by the State Department or intelligence agencies, is scheduled to be released in January.
In late July, Clinton said she was "confident" that she "never sent or received any information that was classified at the time."
She remains the frontrunner to be named the 2016 Democratic contender for the White House, but pollsters say the email scandal has dented public trust in her.