The Internet sites of the IDF, Shin Bet (Israel Security Agency) and Mossad were unavailable to visitors on Sunday, two days after an international group of hackers threatened to launch a cyber-attack on Israel over the navy’s interception of a Gaza-bound flotilla.

Citing the Tehila Internet company, which operates government websites, an IDF spokeswoman said the incident was caused by a “server error.”

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Another two official websites, belonging to the Health and Interior ministries, also went offline.

Despite claims of technical malfunction, suspicion fell on a denial-of-service cyberattack, in which servers are overwhelmed by hits, usually sent from computers infected with a Trojan horse.

The Anonymous hacking group posted a YouTube video on Friday in which it threatened to carry out a cyber-attack on Israeli government sites.

“Greetings, government of Israel,” a computer voice said in the clip.

Referring to the navy’s interception of two vessels carrying pro-Palestinian activists who were trying to break the naval blockade on Gaza, the video claimed Israel was carrying out “piracy on the high seas.”

The group said it would have “no choice but to strike back” if Israel maintains the closure on the Strip.

The navy on Friday intercepted without incident the two-vessel flotilla carrying 27 pro-Palestinian activists.