BRUSSELS (Reuters) - Several NATO websites have been hit by cyber attacks, but they have had no impact on the military alliance's operations, a NATO spokeswoman said.

The attacks, which affected NATO's main website, came amid rising tensions over Russian forces' occupation of Ukraine's Crimea region where a referendum is to be held on Sunday.

NATO spokeswoman Oana Lungescu said on Twitter that several NATO websites have been the target of a "significant DDoS (denial of service) attack." She said there had been no operational impact and NATO experts were working to restore normal function.

(Reporting by Adrian Croft; Editing by Cynthia Osterman)