LONDON—A group of computer hackers claims to have breached NATO security and accessed hordes of restricted material.

The group called Anonymous says it would be "irresponsible" to publish most of the material it stole from NATO but that it is sitting on about 1 gigabyte of data.

Anonymous posted a PDF file Thursday on its Twitter page showing what appeared to be a document headed "NATO Restricted."

Anonymous is a loosely organized group of hackers sympathetic to WikiLeaks. It has claimed responsibility for attacks against corporate and government websites worldwide.

The group also claims credit for disrupting the websites of Visa and MasterCard in December when the credit-card companies stopped processing donations to WikiLeaks and its founder, Julian Assange.