The Anonymous hacktivist collective has taken responsibility for taking down a number of websites Thursday.

The websites of the Israel Defence Forces (IDF), the Mossad, and Shin Bet, Israel's security service are back online after crashing over the weekend, allegedly as a result of a cyber-attack by Anonymous.

The international hackers' group had previously threatened to "strike back" against Israel following the Jewish nation's interception and detainment of an aid flotilla bound for Gaza on Friday.

In an online video, entitled "An Open Letter from Anonymous to the Government of Israel" and posted Friday evening, an electronically generated voice accuses Israel of perpetrating "piracy on the high seas" .

"Your actions are illegal, against human rights, international, and maritime laws," the statement continues. Justifying war, murder, illegal interception, and pirate-like activities under an illegal cover of defence will not go unnoticed by us or the people of the world," the statement says.

If you continue blocking humanitarian vessels to Gaza... then you will leave us no choice but to strike back. Again and again, until you stop. We are Anonymous. We are a legion. We do not forget. We do not forgive. Expect us."

The government-run websites crashed a few hours after the warning was posted, raising suspicions that the malfunction was a direct result of an Anonymous cyber-attack.

But Israeli officials have denied the possibility of an attack, and say that the timing of the malfunction was merely a "coincidence."

Nitzan Miron, a former member of Matzov, the cyber security division of the Israeli military, responsible for defending networks from hackers, said the breakdown was "a really strange coincidence".

Miron, now chief executive of 6Scan, a website security start-up in Tel Aviv, said there had been a hardware crash rather than a software problem caused by a cyber-attack. "Nothing is impossible but it doesn't look like it [a cyber-attack]," he said. Miron claimed a decision to group all the sites in one hardware system had resulted in a chain reaction of malfunctions.