( CNN ) -- Hackers apparently attacked several NATO websites Saturday , but they did not interrupt operations nor was the integrity of NATO 's systems affected , NATO spokeswoman Oana Lungescu said on Twitter . The websites were down for several hours from what 's known as distributed denial-of-service attacks , she said . CNN was unable to access at least three NATO websites hours after the NATO statement . NATO did not say who was responsible , but a Ukrainian website claiming to belong to a group called Cyber Berkut declared in Russian that it originated the attack . `` We , Cyber Berkut , announce that today at 6:00 p.m. we started the cyberattack on NATO resources , '' the site proclaims . `` We , Cyber Berkut , will not allow the presence NATO occupation on the territory of our homeland , because it opposes NATO intervention in Ukraine , '' the statement said . The attack comes the day before a controversial vote on the secession of Crimea from Ukraine . Cyber Berkut is a reference to the riot police known as Berkut , which was used by ousted Ukrainian President Viktor Yanukovych . On eve of vote , Russian troops reported in Ukraine outside of Crimea . CNN team in Crimea : This is a McMoment to remember .
NATO says some of its systems were attacked by hackers . The attack did not affect NATO systems , a source said . A Ukrainian company claims responsibility for the attack .