Hackers brought down several NATO sites over the weekend in what appeared to be a cyber war over growing tensions over Crimea. NATO spokeswoman, Oana Lungescu, said on social networking site Twitter that cyber attacks, which began on Saturday evening, continued on Sunday, although most services had now been restored.

"It doesn't impede our ability to command and control our forces. At no time was there any risk to our classified networks," another NATO official said.

NATO's main public site carried a statement by Secretary-General Anders Fogh Rasmussen saying that Sunday's referendum on Crimea's status would violate international law and lack legitimacy. It was hit by a D0S attack. A NATO-affiliated cyber security centre in Estonia was hit. NATO's unclassified e-mail network was also affected.

A group calling itself "cyber Berkut" said the attack had been carried out by patriotic Ukrainians angry over what they saw as NATO interference in their country. The claim, made at www.cyber-berkut.org, could not be independently verified. "Berkut” were the feared and now disbanded riot squads used by the government of ousted pro-Russian Ukrainian President Viktor Yanukovich.