NATO says several of its websites were targeted by cyber attacks in what appeared to be the latest development in cyberspace reflecting the growing tensions over Crimea.
A spokeswoman for the military coalition, Oana Lungescu, said the attack had "no operational impact" on the alliance.
NATO did not say who was responsible for taking down its websites in a distributed denial of services attack, but a group calling itself "cyber berkut" said the effort had been carried out by patriotic Ukrainians angry over what they see as NATO interference in their country.
Occupied by Russian forces, the largely Russian-speaking Crimea is holding a referendum Sunday on seceding from Ukraine to join Russia. NATO member states have opposed the move, calling it unconstitutional and effectively an annexation of Ukraine's sovereign territory.
Ukraine has been shaken by turmoil that saw pro-Russian president Viktor Yanukovych ousted as anger exploded over his rejection of closer ties with the European Union in favor of Moscow.
Cyber berkut is a reference to the riot police used by the ousted leader.