Cyber attacks on global companies seen as enemies of WikiLeaks drew the attention of US authorities on Thursday and Dutch police arrested a 16-year-old boy suspected in attacks on credit card sites of Visa and Mastercard.

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Internet activists vowed to crash sites that have blocked business with WikiLeaks and PayPal, and others saw sporadic outrages. 

US Attorney General Eric Holder said US authorities were looking into cyber attacks on companies like Amazon.com and others. "We are aware of the incidents," he said.

The teenage boy was arrested by a high-tech crime unit in The Hague after admitting to attacks on the websites of two credit card companies, MasterCard and Visa, the prosecution in the Netherlands said on its website.

The suspect, whose details were not disclosed, was believed to be part of a larger group of hackers under investigation that participated in so-called denial of service attacks, the prosecution said. Data and computer equipment were confiscated during his arrest.

The loosely organized campaign to avenge WikiLeaks against those who have obstructed its operations, calling itself Operation Payback, has already temporarily brought down the websites of Visa and MasterCard, and of the Swedish government.

A succession of US institutions has withdrawn services from WikiLeaks after the website published thousands of sometimes embarrassing secret US diplomatic reports that have caused strains between Washington and several allies.

In Moscow, Russian Prime Minister Vladimir Putin said the arrest of WikiLeaks founder Julian Assange showed the West was hypocritical in its criticism of Russia's record on democracy.

When asked about leaked US diplomatic cables that cast him as Russia's "alpha-dog" ruler of a corrupt bureaucracy, Putin questioned whether the US Foreign Service was a "crystal clean source of information."