Around 150 more of the emails that former US secretary of state Hillary Clinton kept on a private server have been retroactively deemed classified, the State Department says.
The agency Clinton previously headed was set to release later on Monday some 7000 additional pages of the emails 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 say many of these emails 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 emails had also been "upgraded in some form".
Clinton has been criticised 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" - physically located in the bathroom of a private internet provider - 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 unauthorised network that could be prey to hackers.
Clinton, for her part, insists none of the emails 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.
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.