Lance Whitney/CNET

Anonymous is claiming that it has hacked past NATO security and taken restricted documents maintained by the global organization.

"Yes, #NATO was breached. And we have lots of restricted material. With some simple injection. In the next days, wait for interesting data :)," said Anonymous via its Twitter account around 5 a.m. PT today.

As "proof" to back up its claims, the hacktivist group posted two PDF files today that it said it took from NATO. One document was reportedly posted on the site PDFCast but has since been removed. The other document, published on PDF Host.Net is named "NATO-RESTRICTED.Outsourcing-Balkan-CIS.[MCM-0167-2007 (09 Jan 08) - Oursourcing of Balkans CIS Support].pdf," and as of now is still accessible.

Beyond these two documents, Anonymous claimed it was able to grab a lot more, though the group said it would keep most of the material unpublished.

"We are sitting on about one Gigabyte of data from NATO now, most of which we cannot publish as it would be irresponsible. But Oh NATO....," the group announced in another tweet.

A spokesman for NATO told the Telegraph that the organization was investigating the claims by Anonymous.

The reported action against NATO by Anonymous follows a threat issued by the group in early June warning the organization not to challenge it.

In its warning, Anonymous was responding to a report issued in May by Lord Joplin, general rapporteur of NATO, in which he cautioned member nations to beware the rising threat of "hacktivism," or carrying out cyberattacks for political purposes. The report itself singled out Anonymous over the group's cyberattacks against MasterCard, Visa, PayPal, Amazon, and many other companies.

"Today, the ad hoc international group of hackers and activists is said to have thousands of operatives and has no set rules or membership," read part of the report. "It remains to be seen how much time Anonymous has for pursuing such paths. The longer these attacks persist the more likely countermeasures will be developed, implemented, the groups will be infiltrated and perpetrators persecuted."