The group calling itself Anonymous claimed credit Thursday for hacking into NATO servers and stealing 1 gigabyte of sensitive information as part of its campaign to harass and humiliate prominent targets.

The group has attempted to post online some documents collected in the incident and vows to post more soon, but it also said it has decided to withhold some others because posting them would be “irresponsible.” NATO did not confirm the group’s account.

“NATO is aware that a hackers group has released what it claims to be NATO classified documents on the Internet,” Damien Arnaud, a spokesman for the trans-Atlantic military alliance, said in an e-mail. “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.”

Groups calling themselves “hacktivists” — which target Web sites and servers in pursuit of political agendas — have joined the list of cyber threats identified by government and corporate security officials.

“It is one of the up-and-coming biggest concerns for the FBI,” said Robert E. Nickel, unit chief in the FBI’s Public Private Alliance Unit, speaking at a cyber conference last week.

Hackers from Anonymous claim to have attacked NATO and obtained a gigabyte of sensitive data.

“The good news is, when those guys are caught, they fold like a house of cards,” he added. “When someone from the FBI comes and says, ‘You’re looking at 20 years for hacking into the CIA Web site,’ they rat on everybody.”

In May, NATO General Rapporteur Lord Jopling released a draft report on information and national security that mentioned Anonymous as a “one of the most prominent group of on-line hackers.” He stated that the group was becoming more sophisticated and “could potentially hack into sensitive government, military and corporate files.”

Anonymous has conducted denial-of-service attacks to disrupt access to Web sites of PayPal, Visa and Mastercard in retaliation for the companies suspending the accounts of WikiLeaks, the anti-secrecy site that has posted classified U.S. government data.

Anonymous has “declared war on NATO,” said Richard Stiennon, a cyber expert and author of “Surviving Cyberwar.”

The FBI also has targeted Anonymous and related hacker groups. On Tuesday, it arrested 14 suspected Anonymous members for alleged roles in attacking PayPal.

“We anticipate in the long run that with a group like Anonymous, when we arrest, prosecute and sentence them to lengthy prison time, that will help” reduce the activity, Nickel said at last week’s conference.

Anonymous and LulzSec, a related group, issued a message Thursday morning to the FBI, saying “Your threats to arrest us are meaningless to us as you cannot arrest an idea. . . . We have become bandits on the Internet because you have forced our hand. . . . We’re back — and we’re not going anywhere. Expect us.”

Staff writer Hayley Tsukayama contributed to this report.