An internal investigator says senior staff at the US Embassy to Japan, including Ambassador Caroline Kennedy, have used personal email accounts for official business.
Some emails contained sensitive information.
The State Department's Office of Inspector General said that it identified instances where emails labeled "sensitive but unclassified" were sent from or received by personal email accounts.
It says department employees are expected to use approved, secure methods to transmit sensitive but unclassified information when available and practical.
The report on the embassy's operations was conducted between January and March.
Kennedy is the daughter of President John F Kennedy.
The department is currently reviewing thousands of emails of Hillary Rodham Clinton sent and received from a private email account while secretary of state. Clinton also used her own email server.
