Online "hacktivist" group Anonymous claimed Sunday it had stolen a trove of emails and credit-card information from U.S.-based security firm Stratfor's clients, and vowed additional attacks.

Hackers provided a link on Twitter to what they said was Stratfor's private client list, which included the U.S. Defense Department, Army, Air Force, law-enforcement agencies, top security contractors and technology firms like Apple Inc. and Microsoft Corp. MSFT.

They also posted images online claiming to show receipts from donations made by the hackers on behalf of some of Stratfor's clients by using their credit-card data.

The hackers said they were able to obtain the information in part because Stratfor did not encrypt it, which could prove a major blow to a company that calls itself a global intelligence firm.

"Anonymous hacks and discredits @STRATFOR intelligence company," Twitter user YourAnonNews wrote on the microblogging website. "Maybe they should learn what encryption is."

An alleged Anonymous hacker who uses the Twitter handle anonymouSabu claimed that more than 90,000 credit cards from law enforcement, journalists and the intelligence community had been leaked and used for "over a million dollars" in donations.

A widely distributed hacking message posted online, however, mentioned around 4,000 credit cards, passwords and home addresses.

Among the donations shown was a $494 payment on behalf of the Department of Defense for textbooks, a school uniform and food-crisis education provided by charity CARE for impoverished girls and women.

A $180 payment was also made to the American Red Cross on behalf of a Department of Homeland Security official, and was signed "Thank you! Department of Homeland Security." Another $200 payment was made to the American Red Cross on behalf of a Texas Department of Banking official.

In an email to its members, Stratfor said it had suspended its email and servers after learning the website was hacked.

"We have reason to believe that the names of our corporate subscribers have been posted on other web sites," said the email signed by Stratfor chief executive George Friedman. "Stratfor and I take this incident very seriously."

He said Stratfor was "working closely with law enforcement" to identify those responsible for the hack.

By midday, the company's website was down, saying the "site is currently undergoing maintenance."

Wishing a "Merry LulzXmas" to all--in an apparent reference to Anonymous-affiliated group Lulz Security--Anonymous said it would focus its next attacks on celebrities Justin Bieber, Lady Gaga, Kim Kardashian and Taylor Swift.

Anonymous has been involved in scores of hacking exploits, including the recent defacing of a website of Syria's Ministry of Defense to protest a bloody crackdown on anti-government protesters.

Last year, the shadowy group launched retaliatory attacks on companies perceived to be enemies of the antisecrecy website WikiLeaks.