NATO's main page and its cyber defense center was down. A hactivist group calling itself Cyber Berkut claimed to have launched the attacks attacks. The group also claimed to have taken down the site for NATO's Parliamentary Assembly.
The pro-Russian Ukrainian hackers claimed to be responsible for the attacks amid rising tensions over military incursions into the Crimean peninsula.
NATO spokesperson Oana Lungescu confirmed on Twitter that several NATO sites had been the target of a "significant" distributed-denial-of-service (DDoS) attack but said the integrity of the systems was unaffected and experts were working to restore normal functionality.
The attacks came on the eve of Sunday's referendum on whether Ukraine's Crimea region should join Russia. The UN Security Council introduced a resolution declaring the referendum invalid, but Russia vetoed the resolution Saturday.
Along with an escalation in cross-border hacking activity, internet censorship has also seen a recent increase. The Russian government ordered Russian ISPs to block access to a several Russian websites noted for their criticism of President Vladimir Putin and his government.
