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.