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Victims of a hack attack on a US-based security think tank have been warned against speaking out for fears they could be targeted again.

has been down since it was apparently hit by the hacker group Anonymous on Christmas Eve.

The company's chairman used Facebook to apologise to customers but warned against posting messages of support.

The hackers said they would use stolen credit card details to donate "over a million dollars" (£638,000) to charity.

In a, Stratfor thanked users who had offered public support - but warned of reprisals.

"It's come to our attention that our members who are speaking out in support of us on Facebook may be being targeted for doing so and are at risk of having sensitive information repeatedly published on other websites," it said.

"In order to protect yourselves, we recommend taking security precautions when speaking out on Facebook or abstaining from it altogether."

This was followed later with a message.

"On December 24th an unauthorized party disclosed personally identifiable information and related credit card data of some of our members," chairman George Friedman wrote.

"We have reason to believe that your personal and credit card data could have been included in the information that was illegally obtained and disclosed."

Mr Friedman said that a "leading identity theft protection and monitoring service" had been tasked with protecting clients. The company's email and servers have been taken offline.

The company also said the disclosure of what hackers claimed were "private clients" was "merely a list of some of the members that have purchased our publications and does not comprise a list of individuals or entities that have a relationship with Stratfor".

A spokesperson alleging to represent Anonymous claimed they had been able to obtain the information because it was unencrypted.

They claimed that thousands of emails, passwords and credit card details had been taken.

They said Stratfor's clients included the US Department of Defense, law enforcement agencies and media organisations.

However, some confusion has surrounded the perpetrators of the attack. A release also claiming to come from the group on Christmas Day said that Anonymous did not carry out the attack, and in fact supported Stratfor due to its "unbiased" reporting work.

In the past, splinter groups of Anonymous members have emerged, the most notable of which was LulzSec, who were credited with beingresponsible for several high-profile attacks earlier this year.

Anonymous has previously claimed responsibility for cyber attacks on financial institutions seen as enemies of the whistleblowing website Wikileaks.

This latest attack appears to be in support of alleged Wikileaks source Bradley Manningwho currently awaits trial.

A message posted by Anonymous referring to the Stratfor attack read: "We hereby ask that Bradley Manning be given a delicious meal this Lulzxmas, and no, not the "holiday special" in the prison chow hall.

"We want him out on the streets at a fancy restaurant of his choosing, and we want this to happen in less than five hours."

The demand was not met.

At the time of publication, Stratfor could not be reached for comment.