Nato has been fighting against a string of cyber-attacks over the weekend as hackers disabled several of the organisation’s websites, probably in connection with the Crimea situation.
According to Nato’s spokeswoman Oana Lungescu, the cyber-attacks commenced on Saturday evening and continued throughout Sunday, although most services have 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.
The hackers performed the so-called distributed denial of service (DDoS) attack, flooding the websites with requests causing them to slow down or crash. A Ukrainian hacktivist group known as cyber berkut took responsibility for the attack, stating Nato’s interference into Ukraine’s matters as the main reason.
However, the claim of the group, who are supporters of the ousted pro-Russian President Victor Yanukovich, has not yet been independently verified.
The attack also affected a Nato-affiliated cyber security centre in Estonia and unclassified Nato email networks.
Nato's main public website (www.nato.int), which 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, worked intermittently.
During the weekend referendum, 96 per cent of participants supposedly voted for the Crimea region to break away from Ukraine and join Russia.
The website of the referendum (referendum2014.ru) said on Sunday that it had come under a cyber-attack overnight, although it appeared to be working on Sunday.
Cyber-attacks on Nato's computer systems are common, but a NATO official, speaking on condition of anonymity on Sunday, said the latest one was a serious online assault.
Suspected Russian hackers used DDoS attacks to cripple websites and services in Estonia in 2007 during a dispute over a war memorial, and against Georgia during its brief 2008 war with Russia. Moscow denied orchestrating such attacks, saying they were simply carried out by independent patriots.
Groups calling themselves cyber berkut have attacked several Ukrainian websites in recent weeks, computer security experts say.
Cyber berkut – which some experts believe may be affiliated with Russian intelligence – published its statement in Russian rather than Ukrainian.