Several major Russian government websites were blocked by a series of coordinted cyber attacks by hackers this week, officials told the press.

      The websites of the Russian presidential administration, Central Bank, and the Ministry of Foreign Affairs were unavailable for several periods throughout the days on Friday and Monday.

      “A powerful attack on the site is ongoing,” a spokesperson for the Kremlin said on Friday. “Measures are being taken to restore the normal functioning of the site.”

      Bank of Russia said in a statement that its website had been offline for about an hour on Monday morning following a denial of service attack. A source at the central bank told Prime business news agency that the origin of the attack remained unknown.

      By early Tuesday evening all the websites managed to restore their normal work.

      It is still not clear whether those cyber attacks were somehow linked to the current crisis situation in Ukraine and Crimea. An attack on the website of the Russian newspaper Rossiiskaya Gazeta last week was traced to a pro-Ukrainian group of hackers who call themselves the “Maidan Hundred,” a reference to Kiev’s main square where often violent protests broke out in recent months. But this time no hacker group took responsibility for the attacks.

      An official at the Central Bank later said that attack was believed to be linked to the Islamic anti-Russian group Anonymous Caucasus. That hacker group already attacked the Bank's website in October 2013.