Several NATO web sites went down late last week following mounting tensions in the Crimean peninsula. A group of pro-Russian Ukranian hackers has claimed responsibility for the cyberattack, which took place on the eve of Sunday’s referendum.

Hacktivist group Cyber Berkut is reportedly the perpetrators of the attacks which took down several of NATO’s websites. According to NATO spokesperson Oana Lungescu, several of the organization’s sites went down following a “significant” distributed denial of service (DDoS) attack. The sites were not breached during the cyberattacks, and are expected to be restored to normal functionality before long.

The attacks launched by hackers on Saturday were successful in taking down NATO’s main page, as well as the organization’s cyber defence center website. Although hackers claimed to have taken down the NATO Parliamentary Assembly website, this continued to function as usual.

During Sunday’s referendum, Ukrainians voted on whether the Crimean peninsula should join Russia or not, however tensions continue to mount despite a UN resolution.