Cyber activists attacking organizations seen as foes of WikiLeaks briefly blocked a Dutch prosecution website on Friday after a 16-year-old suspected of involvement in the campaign was arrested in the country.
The activists also tried to block the website of online payment firm Moneybookers, but denied their attacks were intended to create business turmoil or badly disrupt online Christmas shopping.
Several companies have ended services to WikiLeaks after it published thousands of secret U.S. diplomatic reports that have caused tension between Washington and several of its allies.
The website continued its release of U.S. cables on Friday, with the latest reports including a prediction by the U.S. ambassador to Cairo that Egyptian President Hosni Mubarak would "inevitably" win 2011 polls and stay in office for life.
U.S. authorities said they had not pressured companies to stop working with WikiLeaks.
"We have not pressured anybody to do anything," Attorney General Eric Holder told reporters in San Francisco, where he was attending a financial fraud conference.
Holder said authorities are aware of the attacks and looking at them, pointing to a computer crimes section of the Justice Department that can "trace back" to where the attacks originate.
Dutch prosecutors said activists targeted their website with "denial of service" attacks that slowed it for several hours and briefly made it unavailable. The incident was probably related to the boy's arrest, they said.
"We have been investigating this with international authorities and we are working together with the FBI," Dutch prosecution service spokesman Wim de Bruin said, referring to the U.S. Federal Bureau of Investigation.
A Rotterdam judge ordered the boy, who was arrested in The Hague on Thursday, to spend 13 days in custody while the investigation continues, the prosecution service said.
The maximum prison sentence in the Netherlands for distributed denial-of-service attacks is six years, de Bruin said.
The suspect, whose identity was not disclosed, told investigators he participated in the attacks on the MasterCard and Visa websites.
The attack on Moneybookers froze the site for about two minutes. Activists said they picked Moneybookers because it informed WikiLeaks in August it had closed its account. They promised to continue their attacks, and spoke of MasterCard and Interpol as fresh targets.
Some participants in a chat room used by the "Operation Payback" campaign were defiant, but others voiced despair at what they considered a lack of discipline.
"The whole thing is getting out of control, people are attacking local police websites and giving us a bad reputation. This was supposed to be to help WikiLeaks and not an excuse for kids to crash random websites," one wrote.
Online retail and web-hosting powerhouse Amazon stopped hosting WikiLeaks' website last week, and on Thursday it briefly became the pro-WikiLeaks campaigners' main target -- before they admitted it was too big for them, for the moment.
The statement by the activists, who collectively call themselves "Anonymous," added that a lack of firepower was not the only reason the attack on Amazon had not succeeded. They felt "attacking a major online retailer when people are buying presents for their loved ones, would be in bad taste."
The Anonymous statement followed one by WikiLeaks, which said the website had no links to the cyber attacks, and neither supported nor condemned them. It quoted WikiLeaks spokesman Kristinn Hrafnsson as saying the attacks were "a reflection of public opinion on the actions of the targets."
Some freedom of information campaigners sympathetic to WikiLeaks say its cause cannot be furthered by denying freedom of information to others. On an online chat service used by the campaign, participants debated whether to end the attacks and focus instead on discovering more embarrassing material in the leaked documents.