The group of computer hackers famous for its support of Wikileaks claims to have breached security at NATO and accessed a bank of restricted data.
Anonymous says it has around one gigabyte of data, but claims it would be irresponsible to publish most of it.
A NATO official confirmed that the hack had taken place and said an investigation was underway.
Anonymous is a loose collective of hackers that has admitted being behind attacks against corporate and government websites worldwide.
A few days ago, the FBI raided homes in New York and California as part of a probe into the group.
Officers say computers were seized and arrests made.