Unidentified hackers brought down several public NATO websites with cyber attacks on Saturday, the alliance said, in what appeared the latest escalation in cyberspace over growing tensions over Crimea.
A group calling itself "cyber berkut" said the attack had been carried out by patriotic Ukrainians angry over what they saw as NATO interference in their country.
The claim could not immediately be independently verified. Cyber berkut is a reference to the feared and since disbanded riot squads used by the government of ousted pro-Russian Ukrainian President Viktor Yanukovich
A number of NATO websites have been hit by cyber attacks, but they have had no impact on the military alliance's operations, a NATO spokeswoman said.
The attacks, which affected NATO's main website, came amid rising tensions over Russian forces' occupation of Ukraine's Crimea region where a referendum is to be held on Sunday.
NATO spokeswoman Oana Lungescu said on Twitter that several NATO websites have been the target of a "significant DDoS (denial of service) attack." She said there had been no operational impact and NATO experts were working to restore normal function.