Unidentified hackers brought down several public NATO websites with cyber attacks on Saturday, the alliance said, in what appeared the latest escalation in cyberspace over growing tensions over Crimea.
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 immediately be independently verified. Cyber berkut is a reference to the feared and since disbanded riot squads used by the government of ousted pro-Russian Ukrainian President Viktor Yanukovich.
As well as the main NATO website www.NATO.int, the website of a NATO-affiliated cyber security centre in Estonia was also affected. The alliance said none of its essential systems were compromised.
Occupied by Russian forces for over a week, the largely Russian-speaking Crimea holds a referendum on seceding from Ukraine to join Russia on Sunday. NATO states have opposed the move, calling it unconstitutional and effectively an annexation of Ukraine's sovereign territory.
NATO spokeswoman Oana Lungescu said the websites had been taken down by distributed denial-of-service (DDoS) attacks.
"Our experts working to restore normal function," she said via the social networking site Twitter. "No operational impact... The attack hasn't affected the integrity of NATO's systems."
She made no comment on where the attack might have originated.
John Bumgarner, chief technology officer at the U.S. Cyber Consequences Unit, a non-profit research institute, said initial evidence strongly suggested that these cyber attacks were launched by pro-Russian sympathizers.
"One could equate these cyber attacks against NATO as kicking sand into one's face," he said.
Tensions between Moscow and the West have been rising steadily since Russia intervened following the ouster of Yanukovich. Ukrainian and Russian websites have both been targets for cyber attacks in recent weeks but this appeared the first major attack on a Western website since the crisis began.
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.
Apparent Ukrainian groups calling themselves "cyber berkut" have attacked several Ukrainian websites in recent weeks, computer security experts say. It was not immediately clear if the attack on NATO was genuinely conducted by the same group.
The group - which some experts believe may be affiliated with Russian intelligence - published its statement in Russian rather than Ukrainian.