Anonymous members claim to have hacked Nato servers, and to have gained access to restricted documents.
The hacker group claimed to have approximately one gigabyte of Nato data in a Twitter post on Thursday.
"Yes, #NATO was breached. And we have lots of restricted material. With some simple injection. In the next days, wait for interesting data :)," the AnonymousIRC Twitter feed said. "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...."
AnonymousIRC published links to documents marked 'restricted' it claimed came from Nato. Nato is looking into the Anonymous claims, a Nato spokesman told ZDNet UK on Thursday. "We're trying to establish the facts," said the spokesman.
One of the documents purportedly carries estimates for Nato command and control information systems costs in Afghanistan totalling over €10m (£8.8m), while the other document allegedly relates to information systems in the Balkans operation.
An AnonymousIRC also asked: "Enjyoing the war yet, NATO? [sic]".
Anonymous and Lulzsec, which police believe are connected, issued a joint statement on Thursday to the FBI following a number of arrests of people with suspected involvement in the groups.
"We are not scared any more," said the Anonymous statement. "Your threats to arrest us are meaningless to us as you cannot arrest an idea. Any attempt to do so will make your citizens more angry until they will roar in one gigantic choir. It is our mission to help these people and there is nothing — absolutely nothing — you can possibly to do make us stop."
In addition, Lulzsec announced that it was going to share News of the World emails with certain news organisations. The group claimed to have gained the emails during an attack which redirected traffic from the Sun website to a fake story of Rupert Murdoch's death.
Update 21-7-11: Nato sent this statement:
"Nato is aware that a hackers group has released what it claims to be Nato classified documents on the internet," said a Nato official. "Nato security experts are investigating these claims. We strongly condemn any leak of classified documents, which can potentially endanger the security of Nato allies, armed forces and citizens."