WikiLeaks supporters wear masks during a demonstration in Malaga
WikiLeaks supporters wear masks of the "Anonymous" internet activist group and a mask of WikiLeaks founder Julian Assange (R) during a demonstration calling for the release of Assange, in Malaga, southern Spain December 11, 2010. REUTERS

Hackers with the loose-knit group Anonymous said on Friday they had broken into the network of U.S. government contractor Mantech International Corp and posted some NATO-related correspondence online.

Anonymous, tweeting as AnonymousIRC, offered the correspondence between Mantech and the North Atlantic Treaty Organization as proof of the breach. Two involve NATO contracting offices, and one discusses deploying staffers to an unnamed "NATO Theater of Operations" for what appears to be tech services.

Mantech, which claims the U.S. Defense, State and Justice Departments among its clients, declined to comment. It offers cyber security among its services.

Hackers associated with Lulz Security and Anonymous have claimed responsibility for cyber attacks on the U.S. Central Intelligence Agency, the U.S. Senate, Sony Corp websites and the website of Murdoch's British newspaper group, News International, among others.

Authorities made some arrests in connection with the breaches, including a teenager detained at a house in the remote Shetland Islands, off Scotland's northeast coast.

There have also been other arrests in Britain and in the United States.

The group has urged supporters to boycott eBay Inc's PayPal electronic payment service, or to close existing accounts. It has previously attacked PayPal to show opposition to the service's refusal to process payments to WikiLeaks, the website founded by Julian Assange that published copies of secret U.S. government diplomatic cables.