The retaliatory attacks by pro-WikiLeaks activists are growing in strength as hackers add botnets and thousands of people download an open-source attack tool, security researchers said today.

n recent days, distributed denial-of-service (DDoS) attacks have been launched against several sites, including those belonging to Amazon,MasterCard, PayPal and the Swiss payment transaction firm PostFinance, after each terminated WikiLeaks accounts or pulled the plug on services.

In a new step in the campaigns, botnets — armies of already-compromised computers that hackers control remotely — are now being recruited for the DDoS attacks, said Beth Jones, a senior threat researcher with Sophos.