The Israeli army and intelligence agencies' websites were offline on Sunday, two days after hacker group Anonymous warned it would "strike back" for Israel's capture of Gaza-bound ships on Friday.

Anonymous, a network of online activists who have attacked government and financial websites around the world, released a statement Friday warning that the group would take action against the navy's seizure of two ships aiming to break Israel's blockade on the Gaza Strip.

"Your actions are illegal, against democracy, human rights, international, and maritime laws," the statement addressed to the government of Israel and posted on Youtube and Anonymous-affiliated sites said.

"Justifying war, murder, illegal interception, and pirate-like activities under an illegal cover of defense will not go unnoticed by us or the people of the world."

Websites for Israel's army, internal Shin Bet security service, and Mossad spy agency, as well as several government ministries, could not be accessed on Sunday. It could not be confirmed that Anonymous was responsible.

An Israeli army spokesman said it was a "strange coincidence" but he could not confirm that hacking was responsible.

"Initial investigations conducted by the internet company indicate problem with the internet servers," a military spokeswoman added. She said they did not know whether it came as a result of a hack.

A government IT official told Israeli daily Ynet that the server malfunction has "nothing to do with an attack, no threat and no hacking."

In its statement Anonymous slammed Israel's deadly raid on a flotilla heading to Gaza last year, which killed nine Turkish nationals.

"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. Again and again, until you stop," the statement said.

Organizers of the 'Russell Tribunal on Palestine' also reported that their website had been hacked on Sunday.

Prominent British lawyer Michael Mansfield, speaking at the international law hearing, noted "I leave you to your own conclusions as to how that has come about."

On Tuesday, the Palestinian internet cable-network was outed in what officials called a "serious act of sabotage."

The Palestinian Authority minister of telecommunications said hackers from more than 20 countries attacked the network, which was restored on Wednesday.