Cyber Berkut

Pro-Russian Ukrainian hackers claimed responsibility for a cyberattack that took down several NATO Web sites amid rising tensions over military incursions into the Crimean peninsula.

A hactivist group calling itself Cyber Berkut claimed to have launched attacks Saturday that took down NATO's main page and that of NATO's cyberdefense center. The group also claimed to have taken down the site for NATO's Parliamentary Assembly.

NATO spokesperson Oana Lungescu confirmed on Twitter that several NATO sites had been the target of a "significant" distributed-denial-of-service attack but said the integrity of the systems was unaffected and experts were working to restore normal functionality.

DDoS attack on some #NATO sites ongoing but most services restored. Integrity of NATO data &systems not affected. We continue working on it — Oana Lungescu (@NATOpress) March 16, 2014

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 handful of Russian Web sites noted for their criticism of President Vladimir Putin and his government.