WASHINGTON - Among the work-related emails that Defense Secretary Ash Carter handled with his personal unsecured account was a message from a group promoting cybersecurity. Oh, the irony.

The Pentagon on Friday released 34 pages of Carter emails that he sent or received using his personal account, a practice that he acknowledged on Thursday had been a mistake, given the well-known and growing threat of cyber intrusions. Carter said he is not a frequent user of email and never used his personal account to handle classified information.

None of the Carter email correspondence released by the Pentagon included classified material, although they covered only the period from April 1 to 30. The emails initially were provided to the New York Times in response to the newspaper's request under the Freedom of Information Act.

Presidential chief of staff Denis McDonough found out in May about Carter's emailing and, through White House counsel, expressed concern to Pentagon lawyers. Carter nonetheless persisted, telling reporters Thursday that he ended the practice "a few months ago."

According to a senior U.S. official, Carter was told about the White House warning, but it was more about following the rules, particularly regarding the preservation of emails he sent using his personal account. When the Pentagon received the Times' Freedom of Information Act request in September, Carter had already reduced his use of the personal account. But the request triggered another assessment by senior staff, who concluded that he should stop, the official said Friday.

The official was not authorized to discuss the matter publicly and spoke on condition of anonymity.

Carter has made cybersecurity policy a high priority. His improper use of a personal email account was especially remarkable given the burst of public criticism that followed disclosures in March that Hillary Clinton had used a private email account to conduct government business while she was secretary of state.

The Pentagon is among the most frequently and intensively targeted government agencies by international hackers.

Carter aides said all of his messages, which were largely about meetings, speeches and other administrative issues, were backed up for record keeping on the department's email system.

The 34 pages of emails released Friday were mainly correspondence between Carter and Eric Fanning, who was his chief of staff at the time. Also copied on many of the messages were other members of Carter's inner circle, including his current chief of staff, Eric Rosenbach, and Ron Lewis, an Army lieutenant general who was Carter's senior military assistant at the time. Carter fired Lewis last month for unrelated and unspecified personal misconduct.

Associated Press writer Lolita C. Baldor contributed to this article.