Militant Islamic group Hamas is urging supporters to conduct a three-day "electronic jihad" against Jewish websites.
The Simon Wiesenthal Center, a human rights organisation which monitors anti-Semitic publications and websites, claims to have found evidence of two Arabic language sites referring to a campaign of hacking into Jewish sites starting on 29 November.
Rabbi Abraham Cooper, an associate dean at the Center, told Reuters that one of the sites gives "how to" tips to would-be hackers.
He said that there had been numerous tit-for-tat hacking attempts between Israeli and Islamic sites in the past two years.
But Cooper warned that there are now areas of the websites devoted to teaching potential hackers how to get involved.
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