File photo of Hillary Clinton.
The US State Department does not know whether more emails are missing from the cache Hillary Clinton handed over last year, which Clinton said at the time was a complete record of her work emails, a department spokesman said on Friday.
"We don't know the degree to which there may be other emails another third party may have ... that we do not have," John Kirby, the spokesman, told reporters.
The department said on Thursday it had learned of at least 15 emails, all on the subject of Libya, that Clinton did not include among the 30,000 she provided when the department asked for her records last year.
Clinton has weathered criticism for her unusual use of her private email address, which was connected to a computer server in her home, for her work correspondence while she was the nation's top diplomat.
The undisclosed emails were all between Clinton and Sidney Blumenthal, an old friend and an informal adviser.
The exchanges came to light after a Republican-led committee of US lawmakers investigating the 2012 attack on diplomatic staff in Benghazi, Libya, subpoenaed Blumenthal's correspondence on Libya with Clinton.
"We only know about these 15 emails because Mr. Blumenthal had them and provided them to the committee, so there was something to check them against," Kirby said.
© Thomson Reuters 2015