IRBIL, Iraq - Defense Secretary Ash Carter acknowledged Thursday that he had used a private email account on his iPhone to conduct some official business at the Pentagon until a few months ago, but insisted he did not put classified information at risk.

"This was a mistake," Carter said here Thursday. "Particularly someone in my position and knowing the sensitivities with this issue, should have known better."

Hillary Clinton's use of a private email server for official business when she was secretary of state during President Obama's first term has sparked an FBI investigation, congressional hearings, and trouble for her presidential campaign.

Sen. John McCain (R., Ariz.), chairman of the Senate Armed Services Committee, called it "hard to believe" that Carter followed the same practice. He said his panel would conduct a review "to ensure that sensitive information was not compromised."

Rep. Mac Thornberry (R., Texas), chairman of the House Armed Services Committee, said the Pentagon's inspector general should investigate whether Carter sent any classified material in unsecured emails in his current job, or in his previous position as deputy secretary of defense from October 2011 to December 2013.

It wasn't immediately clear if Carter's use of his private email account for official business violated Pentagon policies or other federal regulations..

On a visit to U.S. troops in this northern Iraqi city, Carter said he had sent work-related emails from a private account on his iPhone. He said he did not stop until "a few months ago."

Carter said the messages were "administrative" in nature and did not include classified material. He said copies were backed up on Defense Department computers.

Hackers and foreign intelligence services routinely target email accounts of senior government officials, especially those using unsecured servers, and have stolen vast amounts of data. The Pentagon has made cybersecurity a top priority.

Administration officials said White House chief of staff Denis R. McDonough learned of Carter's practice in May and directed the White House counsel's office to contact the Pentagon to make sure Carter was following rules regarding digital correspondence.

The White House appeared to downplay the issue Thursday, saying Carter had acknowledged a mistake and rectified it.

The Pentagon confirmed Carter's use of private email earlier Thursday after the New York Times reported that it had obtained 72 emails that Carter had sent or received from his private account. The emails, released under a Freedom of Information Act request, were between Carter and his then-chief of staff.