The websites of Israel's army and intelligence services were down Sunday, two days after a hacker group appeared to threaten the Jewish state over its interception of a Gaza-bound flotilla.
The websites of the Israel Defence Forces, Shin Bet domestic intelligence agency and the Mossad spy agency were all down.
"We can confirm that the website has been down for several hours and an investigation is ongoing," a spokeswoman for Shin Bet told AFP.
The Israeli army also confirmed the IDF site had been down for several hours, but said there was no information on why.
"Initial investigations conducted by Tehila, the Internet company, indicate problems with the website servers," an IDF spokeswoman told AFP.
The website outages came two days after a video apparently from "hacktivist" group Anonymous was posted on YouTube, threatening the Israeli government with retaliation over its interception of two Gaza-bound ships.
The boats carrying 27 activists, crew and journalists were intercepted in international waters before they could breach Israel's blockade on the Palestinian territory.
The flotilla was the second time activists have tried to run the blockade since May 2010, when Israeli commandos raided a Turkish-led group of ships, killing nine Turkish activists and sparking a diplomatic crisis with Ankara.
The video, entitled "An open letter from Anonymous to the Government of Israel," accuses the Jewish state of "piracy on the high seas."
"If you continue blocking humanitarian vessels to Gaza or repeat the dreadful actions of May 31st, 2010 against any Gaza Freedom Flotillas then you will leave us no choice but to strike back," the video warns.
It was not immediately possible to confirm whether the video was posted by the hacker group, which has claimed involvement in various hacker activity, including most recently exposing details of individuals who visited child porn sites.
And it was not clear whether the group was directly responsible for the website malfunctions, with Israeli officials declining comment on the issue.
Anonymous has been involved in scores of hacking exploits including the defacing of a website of Syria's defence ministry to protest a bloody crackdown on anti-government protesters.
Last year, the shadowy group launched retaliatory attacks on companies perceived to be enemies of the anti-secrecy website WikiLeaks.
On Tuesday, a day after the Palestinians won full membership of UNESCO, over US and Israeli objections, hackers from around the world attacked Palestinian servers, cutting Internet service across the West Bank and Gaza.
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