Several NATO websites suffered "significant" denial-of-service attacks, NATO spokeswoman Oana Lungescu said Sunday on Twitter.
The attacks caused no operational impact, Lungescu said.
"Several #NATO websites have been target of significant DDoS attack. No operational impact. Our experts working to restore normal function," Lungescu tweeted.
She later tweeted: "@IvoHDaalder pls see my tweet on DDoS attack. No operational impact. Working to restore function."
"The attack hasn't affected the integrity of #NATO's systems. And as @IlvesToomas tweeted, http://ccdcoe.org is back up," the spokeswoman said.
Lungescu said Sunday most service on NATO websites had been restored.
"DDoS attack on some #NATO sites ongoing but most services restored. Integrity of NATO data &systems not affected. We continue working on it," she tweeted.