NATO said Thursday one of its websites was the subject of a probable data breach, but it did not contain any classified data.
A NATO statement said access to NATO's public e-Bookshop, which is operated by an external company, had now been blocked.
Police dealing with cyber crimes had notified the alliance of a probable data breach, the statement said without providing details.
NATO'S e-Bookshop is a separate service for the public for the release of NATO information and does not contain any classified data, the statement said.
The 28-nation Western military alliance has made defense against computer hackers one of its top priorities and news of the probable attack came after NATO defense ministers agreed this month on a cyber defense action plan.
NATO officials say the policy remains classified, but focuses on protection of the alliance's own computer networks and defense planning process, with the aim of bringing all NATO bodies and command structures under centralized protection by 2012.
An increasing number of network break-ins have been reported by institutions and companies in recent months. Those hit recently have included the International Monetary Fund, Lockheed Martin Corp, the biggest information technology provider to the U.S. government, Citigroup Inc, Sony Corp and Google.
(reporting by David Brunnstrom, editing by Rex Merrifield and Angus MacSwan)