The US State Department said on Monday it had retrospectively "upgraded" the status of Clinton's work emails to classified, after it wasn't clear at the beginning whether the correspondence was confidential or not.

Officials on Monday said they categorized 150 additional work emails as secret, after announcing they were making 7,000 more pages of her email correspondence available to the public.

The latest announcement brought the total number of currently classified emails to a little over 200. Last month, officials said 63 other emails had been "upgraded in some form," meaning that sensitive data in the messages had been redacted.

"It's not an exact science," US State Department spokesman Mark Toner said, explaining why the department had decided to classify the messages in hindsight. "When we've upgraded, we've always said that that certainly does not speak to whether it was classified at the time it was sent," he added.

Clinton is believed to have received a total of 62,320 emails during her term in office between 2009 and 2013. She used her private email account connected to a server in her New York home to exchange official messages, although she claims she neither sent nor received any confidential data through email during that period. Around 30,490 of these messages were handed over to the State Department for public release, while the rest were deemed to be personal records.

Officials of the US government are forbidden to transmit classified information outside secure, government-controlled networks. Critics say that the current controversy has dented public trust in Clinton, who is a candidate for the Democratic nomination in 2016's presidential election.

mg/jm (AFP, Reuters)