The US Air Force is notifying more than 33,000 officers that their personal data has been potentially stolen by a hacker.
The hacker is said to have used a legitimate user's ID and password to access personal information held on officers contained in the force’s Assignment Management System (AMS).
AMS is an on-line system that includes career information, birth dates and social security numbers.
So far, there has been no indication that the information has been used to commit identity theft fraud, said the US Air Force.
The breach is believed to have taken place between this May and June, and is being investigated by the Air Force's Office of Special Investigations.
The Air Force says it has strengthened the security on its systems since the breach. There are around 70,000 officers in the force.