He also said that there was no indication that Kennedy had violated department policies.

The State Department was implementing all the recommendations in the report, Kirby said. There was no separate statement from Kennedy about the matter on the US Embassy in Japan's website.

"Department policy is that employees generally should not use private email accounts (for example, Gmail, AOL, Yahoo, and so forth) for official business," the report said.

"Employees are also expected to use approved, secure methods to transmit sensitive but unclassified information when available and practical."

As is typical of such inspector general reviews, the report was largely critical, although former senior US diplomats said it was by no means the harshest that they had seen and that many of the issues raised were familiar.

However, the report noted that, in a departure from State Department practice, Kennedy's chief of staff attended meetings as a note taker but that there were "gaps" in the record of what was discussed.

State Department emails have become an issue in Clinton's campaign for the Democratic presidential nomination because as secretary of state she used a private "clintonemail.com" email address rather than an official "state.gov" one.

Clinton has said the unusual arrangement broke no rules that were in force at the time, although the arrangement has caused long delays in providing federal records to lawmakers and the public to which they are entitled, critics say.