Defense Secretary Ashton Carter used his personal email for government business in the early part of his tenure, a spokesman said, triggering concerns that hackers would access sensitive information.
The revelation comes in the wake of the uproar over Hillary Clinton's use of a personal email account while serving as secretary of state. That case became a Republican talking point as the former chief US diplomat seeks the Democratic nomination for president in the 2016 election.
The practice was first reported yesterday by The New York Times, which quoted an administration official as saying White House Chief of Staff Denis McDonough had had the Pentagon explain why Carter was relying on his personal email account. Carter has been in the post since February.
The practice likely violated a 2012 Defense Department ban on employees using personal email accounts for government business.
In a statement, Carter spokesman Peter Cook acknowledged the use of a private email account was wrong.
"After reviewing his email practices earlier this year, the secretary believes that his previous, occasional use of personal email for work-related business, even for routine administrative issues and backed up to his official account, was a mistake," Cook said.
"As a result, he stopped such use of his personal email and further limited his use of email altogether."
This would make Carter the second top official in President Barack Obama's administration to have used a private email account for work.
Although nearly all senior administration officials occasionally use a personal email account to communicate about work, officials told the Times that Carter had emailed close aides about speeches, meetings, interviews on the news and other matters.
But Cook, the spokesman, minimized Carter's use of his personal email.
"Secretary Carter strongly prefers to conduct communications on the phone or in person, and like many of his predecessors rarely uses email for official government business," he said.
"The secretary does not directly email anyone within the Department or the US government except a very small group of senior advisers, usually his chief of staff."
Cook also stressed that "any email related to work received on this personal account, such as an invitation to speak at an event or an administrative issue, is copied or forwarded to his official account so it can be preserved as a federal record as appropriate.
