The computer hacker group Anonymous on Thursday claimed to have broken into NATO servers and acquired "lots of restricted materials.", DPA reported.
It published eased two documents online appearing to stem from committees of the military alliance's council of ambassadors.
"Yes, NATO was breached," Anonymous wrote on the micro-blogging site Twitter. "And we have lots of restricted material ... In the next days, wait for interesting data."
"We are sitting on about one gigabyte of data from NATO now, most of which we cannot publish as it would be irresponsible," the hacking group added later.
It did post a 36-page document dated August 2007 - titled "CIS Support for New HQ ISAF Joint CIS Control Centre" - along with a 10-page document dated January 2008 and titled "Outsourcing of Balkans CIS Support."
CIS typically stands for communication and information systems. ISAF is the NATO-led mission in Afghanistan.
Both documents feature NATO logos and "NATO Restricted" headers, but their validity could not immediately be verified. They were released on a Belgian national holiday, with NATO headquarters closed.
A NATO official said: "NATO is aware that a hackers group has released what it claims to be NATO classified documents on the internet. 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."
Anonymous has been linked with numerous denial-of-service internet attacks, which incapacitate websites by bombarding them with queries from networks of hijacked computers.