The loose collective of hackers known as Anonymous say they have hacked Nato and accessed an enormous cache of data, documents and restricted material from its servers - but will not release it as it would be "irresponsible".

At about 13:00 on Thursday a tweet from the group's @AnonymousIRC account 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..."

Apparently to prove that it did in fact have access to the data, the group directed users to a PDF file that contained apparently restricted information from the military alliance.

"We have lots of restricted material. With some simple injection. In the next days, wait for interesting data," the group said via Twitter.

Later the group sent out a link to another document, but the site upon which it was hosted quickly crashed.

At around 16:00 the group tweeted again: "Hi NATO. Yes we haz more of your delicious data. You wonder where from? No hints, your turn. You call it war; we laugh at your battleships".

Nato said they were monitoring the hack. An official there 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 also sent out a joint-statement with the separate hacking group LulzSec in which they made a statement to the FBI: "Governments and corporations are our enemy," the statement says. "And we will continue to fight them."

The leaks came on the same day that LulzSec claimed to be about to release emails from a 4GB cache they said they took from the servers of The Sun newspaper on Tuesday, when the News International paper was hacked and replaced with a story announcing the "death" of Rupert Murdoch.

The group later pulled back from the leak because they said it might compromise an ongoing criminal investigation. It now says that it may release some of the information in stages.

"We're currently working with certain media outlets who have been granted exclusive access to some of the News of the World emails we have," said a tweet from the official @Lulzsec feed.

Security experts said that while Anonymous and LulzSec had made headlines, their methods were nowhere near the level of those employed by hackers working for governments and other major organisations.

Labelling the group "16-year-olds" working from their bedrooms, Justin Crump, CEO of Sibylline, an internet security firm, said that Nato probably had little to fear.

"It's still nothing important, they're still using basic methods," he said. "Frankly this is nothing compared to what the Chinese do. That's what I'd be worried about. The Chinese you won't even know they're in there."

Writing later, Crump added on The Huffington Post:

"NATO's last loss was a compromise of the on-line book store - hardly a matter of national security, although nonetheles­s embarrassi­ng. As for the age comment, the issue in general is that these chaps are a bit gutsier in cyberspace than they are in real life - and the unravellin­g of LulzSec shows this pretty well. Nonetheles­s, the quantity they can bring to bear has a quality all of its own and the Anonymous movement has in general been very successful in empowering people to act as 'hacktivis­ts'."