Spokeswoman for the NATO military alliance, Oana Lungescu, said on Twitter that several of its websites had been hit by "a significant DDoS [denial-of-service] attack" on Saturday.
But it said the attacks had no operational impact.
In DDoS attacks, hackers hijack multiple computers to send a flood of data to the target, crippling its computer system.
Lungescu said experts are working to restore normal function, but the websites had remained down for hours and still could not be accessed early on Sunday.
Lungescu did not say who was responsible for the attack, which was claimed by a Ukrainian hacker group called Cyber Berkut, the name given to the feared elite riot police involved in a bloody crackdown on protesters in Kiev.
In a statement on its website, the group said it had targeted three NATO websites over what it claimed was the alliance's interference in Ukraine and support of the "Kiev junta".
"We will not allow the presence of NATO in our homeland," said the statement, which could not be independently verified.
The electronic attack is the latest of several that have seen Ukraine tensions hit cyberspace.
On March 8, British-based BAE Systems said dozens of computer networks in Ukraine had been infected by an aggressive new cyberweapon called Snake, which experts said was most likely the work of Russian hackers.