US Defence Secretary Ashton Carter admitted on 17 December that he used a private email account on his iPhone for official Pentagon business, calling it a "mistake". Carter insisted that the emails were administrative in nature and did not contain classified information.

"This was a mistake," Carter said. "Particularly someone in my position and knowing the sensitivities with the issue, should have known better." He added, "It's not like I didn't have the opportunity to understand what the right thing to do is. I didn't do the right thing. This is entirely on me."

The admission comes as former Secretary of State, and Democratic presidential frontrunner, Hillary Clinton faces continued criticism for her use of a private email server for official business. Clinton has been subjected to an FBI investigation and congressional hearings for her email scandal.

Republican Senator John McCain, the chairman of the Senate Armed Services Committee, said it was "hard to believe" Carter followed the same practice as Clinton, given the backlash she has faced. "With all the public attention surrounding the improper use of personal email by other administration officials, it is hard to believe that Secretary Carter would exercise the same error in judgement," McCain said in a statement, according to the Los Angeles Times.

McCain also announced his committee would review the emails "to ensure that sensitive information was not compromised". Carter was visiting US troops in Irbil, Iraq when he revealed he had sent administrative emails from his iPhone and that he did not stop until "a few months ago." The Pentagon chief said copies of the emails were backed up on Defence Department computers.

The Pentagon acknowledged Carter's use of private email after releasing 72 work-related emails Carter sent or received from his private email account. The emails, released under a Freedom of Information Act request by the New York Times, included messages about legislation, speeches, meetings and TV appearances between Carter and his then-chief of staff, Eric Fanning. Fanning is now the new secretary of the US Army.

"After reviewing his email practices earlier this year, the [defence] 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," Pentagon spokesman Peter Cook said in a statement. "As a result, he stopped such use of his personal email and further limited his use of email altogether."

Cook added Carter "has a secure communications team that handles his classified information and provides it to him as necessary. Memoranda are provided to him in hard copy. He takes responsibilities with regard to classified material very seriously." It remains unclear how many unsecured emails Carter sent before stopping and whether his use of private email for official business violate Pentagon policies or federal regulations.

According to The Washington Post, White House Press Secretary Josh Earnest said that Carter had amended his mistake when asked about his email practices. "He indicated that he didn't frequently use personal email for government work," Earnest said. "He says that those emails did not jeopardise the proper protection of classified information. It clearly is a mistake because it runs counter to our policy."